Powder foundations cost around $10 to $15 dollars. In terms of picking out a shade, make sure you have a mirror handy. Pick out two to three colors that look close to your skin tone, and compare it side by side to your face. Since the colors are close, it will force you to pay attention to the slight differences.
Also, I happen to be a dark skin African American. So, I am always for the companies that have a wide range of color selections. Personally, I recommend Loreal True Match, and Revlon Colorstay lines.
It ranges up to about 15 dollars for the nicer drugstore foundation. With powder it's really hit or miss, since you can't open it and test it on yourself. If you want, you can buy a few of the cheapest ones you can find, then take them home and test them there, then go back and find a more expensive one (if you want to) that's similar in colour.
Ulta also makes a pretty nice and inexpensive ($14, so not much more than what you'd find in a CVS or a target) mineral foundation, and they have specialized beauty helpers there that you could ask for an opinion on what tone you should get.
Normally, drugstore brand powders can vary from €6-€10 (9-15 US dollars).
Because you can't test them on yourself, I would suggest advising with the cosmetologist who works at the drugstore; they will be able to tell you which brands would be best suited for your skin type, and which shade works the best for you.