Check my previous post, I included a case, the Antec Three Hundred for $60. It's a pretty well rated case and also what I have personally. If you have something similar to a MircoCenter computer store near you, I've seen them as cheap as $40, which is how much I paid for mine.
Building a computer is much easier than you probably think it is. If it's something you're interested in, check out r/buildapc
for some really good information.