The buying of your monkey is just a beginning. You will have to build a large cage even if you buy a small monkey. Don't even think about buying a parrot cage. They are really to small. You also need to provide cooling, heating and lightening.
Monkeys are very social animals.
If you are not home all the time, you will have to buy a companion for him. Realize when you buy a monkey that it will be nearly impossible for you to travel since it is very hard to find somebody who can and will take care of your monkey. If you are going to let your monkey run around in your house you will have to keep an eye on them all the time. All monkeys bite. I guess it is needles to say that a baboon will bite a lot harder than a marmoset.