I know what you mean about hoping that the case would be one of the more affordable pieces, I thought the same thing when I built my first computer. But you should really make sure you get a quality case.
If you go cheap with the case it can lead to a lot more pain in the big picture, namely heat and noise, but also the cheaper cases just don't last as long, and sometimes literally fall apart!
Here are a few that I found though:
HAF 942 (It's a full tower, probably way too much for what you want, goes at like $180)http://www.newegg.com/Product/Product.aspx?Item=N82E16811119225&cm_re=942-_-11-119-225-_-Product
But here is something you could probably use though(Mid ATX, USB 3.0, and cheap):
Thermaltake Armor A60http://www.newegg.com/Product/Product.aspx?Item=N82E16811133181