Sure. You need a scale, but high precision isn't terribly important if you mix up large batches at once. A postal scale probably won't cut it though; you want at least as good as 100mg precision. You can get these ingredients from online retailers (except the orange extract — get that at a grocery store). I've used Hard Rhino Muscle and Pure Bulk and been satisfied with both.

My caffeine/theanine formula:

Caffeine - 1.6g

L-theanine - 3.2g

Water - 2 cups

This makes for 100mg of caffeine per fluid ounce of solution, which makes measuring pretty easy. That is like a cup of coffee on the weak side of average, or 2-3 cups of tea. 200mg is like a strong cup of coffee. I have found that 100mg twice a day is close to the sweet spot where I get energy from it but don't build a tolerance. Obviously if you already have a tolerance, you'll want to use more. This mixture should be refrigerated to keep stuff from growing in it. Also, you can vary the concentration a little, but there is a limit at which the theanine solution will gel into an unusable colloidal mass. Hot water is useful for getting the ingredients to dissolve initially.

The theanine is there for two reasons: 1) it smooths out the jitters from caffeine, and has a mood elevating effect when used with caffeine and 2) studies indicate that combined with caffeine, it has a number of cognitive benefits. 2:1 theanine to caffeine is around the ratio that evidence supports.

My flavoring solution is:

Citric acid - 30g

Malic acid - 10g (optional)

Sucralose - 1.5g

Orange (or lemon) extract (83% alcohol) - 8g

Water - 2 cups

I put this in a squeeze bottle like the kind fast food places sometimes put vinegar and oil in, and it only takes a little squirt (about a teaspoon) to flavor a cup of water. I don't bother measuring this, as it's easy enough to do by taste. But do be careful not to make it too strong or the citric acid will hurt your mouth. Nothing dangerous, but it can make your gums sore.

Malic acid is not strictly necessary. It adds a bit of fruitiness, but it's subtle. You can leave it out and use 35g of citric instead of 30g.

Sucralose is the sweetener in Splenda, but it's important to remember that Splenda is highly diluted with fillers to make it the same sweetness as sugar. Pure sucralose is 600 times as sweet as sugar, so it must be measured with care. Hard Rhino is the only source I've found for it. Other sweeteners like xylitol, stevia extract, and erythritol are easier to find. Xylitol and erythritol are not sweeter than sugar, and I find stevia unpleasant. If you like stevia, it's actually more cost effective than sucralose; it's half as sweet, but it's cheaper.

If you don't want to bother with the flavoring solution, you can also mix the caffeine with anything that is sweet and sour because those flavors mask the bitterness of the caffeine. Citrus is particularly effective.

These are the most cost effective ways I've found to have tasty beverages and caffeine. The caffeine solution comes out to about 7 cents per reconstituted cup (18 cents for the equivalent of a Starbucks Grande). Most of that is actually the l-theanine; if you forgo that, it's only half a cent per cup.

The flavoring comes out to under two cents per cup, or $0.30/gallon, but depends on how much you use.

So basically, if I take this with me on a trip, I can have the same caffeine boost as a Grande, plus theanine to take away the jitters, for twenty-five cents. And as an added bonus, it's actually drinkable.

Edit: One more thing. You can use volumetric measurements if you can find the bulk density for your powders, but I don't recommend it. The scale I have is an American Weigh DIA20, which costs about 20 bucks. The only thing I don't like about it is that its maximum capacity is 20g, but it's easy enough to measure the citric acid in two shots.