There's a lot of great suggestions in this thread:
https://permies.com/t/22355/toxin-ectomy/sleep-Mattress-Alternatives. Everything from affordable latex mattresses, to wool mattresses, to making your own mattress out of wool or rocks or straw or ropes or hemlock boughs.
As for my own experience, I bought my son a twin latex mattress for $295 at
http://www.mattresses.net/labeforki.html. They use pieces left over from when they make larger beds, but I've never been able to tell any difference. It's really comfortable! Far more comfortable than my toxic memory foam. They also sell full mattresses made the same way for $380, as well as move conventionally priced (i.e. expensive) mattresses.
I hope that helps!