I don't know about specific recommendations associated with price, but instead, I recommend keeping an eye on Sentai Filmworks' website. They often have really good sales on complete series and movies, of which sometimes can go down to $5.
Outside of that, if you're lucky, check out your local thrift stores, Goodwill's, and so on. They sometimes have random anime for cheap there too. You just can't be too picky. In fact, it's a good way to find diamonds in the rough!