There are plenty of cheap yet decent foam mattresses on Amazon. Check out the "Lucid" ones. They have a variety of firmness to choose from ... I don't use it as a daily sleeper but slept on one recently and it was quite nice. Plus it's one fifth of the price of Casper or traditional inner spring ones.

If you really want to score a deal on one, find the one you want and track it via CamelCamelCamel. The prices change daily and if you're patient, you can get it for 30-35% less than the top line price.