Advice needed: buying a new mattress

So here's the kind of conundrum that makes me wistful for the caveman days of old when we had to sleep on the ground. I'll bet we were just darned happy we knew how to rub two sticks together to make fire and had recently skinned a woolly mammoth so we had something to snuggle under...

Here's Jonah Lisa's question:

Wondering if you guys can answer a quick question. What do you do about beds for your little treehuggers? I'm moving my almost 3 yo from a conventional baby crib (with one of those normal crib mattresses) to a twin. I'd love to be able to put him on a 100% organic, chemical free mattress but even twins are WAY expensive. I just don't have the money for it right now.  What do you guys do? Did you splurge for the eco mattress or go with a cotton futon or encase whatever you use in a pricey (but not as pricey as the eco mattress) wool pad? Or did you go with a conventional mattress and pad and just try not to think about it, as I did with the crib? Any input/insight will be helpful.

Readers, we're counting on you. Any suggestions on buying (or not buying) an eco mattress?