Source: https://www.ufopaedia.org/index.php/Economy_(EU2012)
Timestamp: 2019-04-24 12:28:43+00:00

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Besides XCOM's fixed costs, the Commander will require credits to hire soldiers and interceptors, build items, armor and vehicles, construct base facilities, and develop Foundry projects.
2.1 Do I Really Need That?
There are three main ways of directly raising credits (§).
The first source of income in the game is to expand satellite coverage. Because you get income from satellites at the end of each month and Satellite Uplinks and satellites have long construction times, you will benefit greatly from some planning to make sure you can launch just before month end, rather than just after. Remember to factor in the engineers and power you'll need and make sure you build Generators and Workshops with enough lead time. Your Satellite Uplinks should be your first priority for adjacency bonuses, because an extra satellite is considerably more valuable than +2 power or a Workshop rebate. You will need 16 satellites to cover the whole world (or 8/9 to prevent losing the game), so with 4 Satellite Uplinks and a Satellite Nexus (or 2 Uplinks and 2 Nexuses) you will need 4 adjacency bonuses.
While Managing Panic will be your first priority with satellites, fully covering Africa should be a major priority for the 30% Situation Room increase in global income.
The Gray Market is your second major source of income by selling alien artifacts. While selling things you might need later is a painful decision, it's almost unavoidable to be able to afford to equip your soldiers properly while keeping up with satellite deployment.
Sectoid, Thin Man and Floater corpses are usually good bets in the early game, since you will usually get more. You should stop selling Sectoid corpses around month 3 and Floater corpses around month 4, because you stop seeing those alien types and you may want the corpses for the interceptor consumables. You'll always have a supply of Thin Men from Council missions.
While it may be tempting in the first month or two, you should never sell Weapon Fragments because they're the main limiting factor on Research and Foundry projects.
While less crucial, it's easy to run out of Alloys and Elerium to manufacture late game armor, so also avoid selling those if you can.
After researching them, UFO Power Sources can be good things to sell in the early game. UFO Power Sources are mainly used for Firestorms, but Interceptors with Plasma Cannons will suffice until late in the game, and are otherwise only used for Elerium Generators, which are pure luxuries given Thermo Generators can easily fulfill your power needs. UFO Flight Computers are similar but are used for Satellite Nexuses, which are useful for completing your satellite coverage, so think a little harder before selling them.
If you have the Slingshot DLC, and complete all 3 missions on the first try, you get a massive supply of Elerium, Alloys, and a few Computers, Generators, and two Fusion Cores upon completion of the third: you can easily sell a good portion of each on the Grey Market, keeping you well funded quite early on, and not be hurting on these resources later.
The third way to raise credits is by successfully completing Abductions or Council missions.
Abduction missions can give rewards of either §200 or Scientists/Engineers/Soldiers.
Council missions are similar but the amount of credits given will vary.
Once or twice per month a member nation of the Council will ask for XCOM to supply them with certain items.
Occasionally they will ask for a satellite to be placed over their airspace; this must be done through the normal Launch Satellites screen.
More usually they will request items from the Gray Market list or that XCOM can manufacture (Laser Rifles, Arc Throwers, etc). This is the only time that items from the latter category can be sold.
In exchange for the items or satellite, the Council nation will provide XCOM with a single reward similar to those offered for Abduction missions. (i.e. the nation will offer one of either Scientists, Engineers, a veteran soldier or credits).
If credits are offered, it will always be greater than the normal going rate for the items. For items that can be manufactured, the cost will always be high enough that you can make a cash profit by building the items to order, but the cost of components (Weapon Fragments, Alloys, etc) needs to be considered.
In addition to the economic gain, fulfilling these requests improves your score at the end of the month.
The advice from the section above on general use of the Gray Market also applies here.
If you don't mind save-scumming: loading from at least a little before the request is made, the requested item and reward changes each time you load. Obviously, best done for profit, but also if the item they're asking for isn't something you aren't too keen on giving up, or don't have the resources to make.
Only buy or build what you need rather than what you want.
You can manufacture soldier equipment instantly, even just before a mission, so there's no need to stockpile. If you find you need an extra Nano-Fiber Vest for your squad setup just back out, manufacture one and select the mission again.
There's little reason to have more than one of each interceptor consumable in your stores. When you use one, manufacture a replacement instantly. Even if you need to use two interceptors in quick succession, you can still easily manufacture another between launches.
Take the time to remove items and armor from troops to re-equip them on whoever is going out on the mission. It may take time, but it will save resources in the long-run. Note that injured soldiers who go to the Infirmary automatically remove all their special gear, but soldiers undergoing Psi testing do not.
Other equipment as required or up to your personal preference when equipping troops.
Finally, soldiers' equipment can never be sold at the Grey Market but weapons and some items can be requested by Council members. Use those to get rid of obsolete items, although some items like Laser Sniper Rifles will never be asked for.
Some projects can be pure luxuries to invest early. Think carefully if you really need that item/upgrade before spending resources on it.
Rapid Recovery in the Officer Training School is a prime example. It's available early, but it's quite expensive, and you can always swap in a new Rookie or Squaddie to get some experience.
If you have more Engineers than required to build items then the item cost will be reduced up to a maximum of 50%. Plus if you have Workshops adjacent to one another, for each adjacency (marked with a + on the Facilities overview) you'll get a 7% refund on the credits used after you've built the item.
4 Workshops in a 2x2 configuration (the best configuration) will give a 28% refund. If you are building 6 Titan Armors, with a base building cost of §150 each, you'll receive §252 back. The downsize is that 4 Workshops cost §600 to build (plus the power facilities required) and add §104 on permanent monthly maintenance costs so it is better to plan their construction so that they'll be online only when you start manufacturing the high cost items. The Europe continental bonus reduces the Workshop building cost to 50% so it is critical to get the most savings out of 4 Workshops.
If you are considering 9 Workshops on a 3x3 configuration, think carefully. They'd cost §1350 (excluding excavation, Access Lifts and power facilities) to build and have a monthly upkeep cost of §234. You'd get a massive 84% credit rebate on building items but with so many Engineers, the cost of 6 Titan Armors would already be reduced to 50%, or §300 at start, so you'd receive §252 back. But with the cost, base space, time and power required to build and them, this means a huge investment.
Finally, since you can't sell the items produced in Engineering in the Gray Market, there's no way to keep the Workshops always occupied and get a credit return, since you can only sell their output with Council requests.
Overall, if you can spare the resources and get the Europe bonus, Workshops can be a sound investment, specially if you use the adjacency bonuses. Otherwise stocking up in Engineers might suit better your needs.
Money: Look at the Funding table.
If you choose to start in the USA, you get §180 each month to start the game, in addition to the Base Funding depending on your chosen difficulty level.
If you start in Africa, you actually get an extra 30% to Nigeria's funding and an extra 30% of the Base Funding thanks to the "All In" bonus; plus, you'll be getting an extra 30% from every other funding source as you expand satellite coverage.
Bonuses: This depends on your play style and gameplan.
If you really want Future Combat and We Have Ways early in the game, put your base in Asia because it will only take 2 satellites to get S. America vs. 4 satellites if you start in S. America and expand to Asia.
Autopsies and interrogations don't take very long, so We Have Ways is probably the one clear-cut choice to not start your base, especially considering you only need 2 satellites to get it through the course of the game.
Panic: Face it, you can't respond to all 3 Abduction missions.
At some point, you're going to get stuck with covering a country with a satellite just to calm them down, whether it's early in the game to keep everyone happy or late in the game to prevent completing the Situation Room.
You can try to keep an extra Satellite and Uplink slot available for such cases, though not utilizing such a vital resource also weights on the mind.
Start in Asia (China) for the Future Combat Bonus, then spread to either N. America (Total §330 + Air & Space) or Africa (Total §325 + All In); either way it's only 3 satellites.
This is a good option if you rely on a short-list of highly perked soldiers for combat.
The downside is you have to be careful not to waste precious resources on excessive Foundry projects that you may not use in combat very often.
Start in Africa for the All In Bonus (because it gives you extra credits no matter what play style or strategy you choose) and then spread to N. America for more credits or Asia for even cheaper deals on Foundry and OTS projects.
This is a good option if you don't have a particular strategy in mind, or if your strategy is to play it by ear and react to what the game gives you as you progress.
The downside is that you'll need a couple more satellites to get the other continent bonuses, which takes time, credits, and Satellite Uplinks to accomplish, all in the face of ever-increasing panic.

References: §200
 §150
 §252
 §600
 §104
 §1350
 §234
 §300
 §252
 §180
 §330
 §325