My husband and I found the solution for us for the poison-free, comfortable, inexpensive mattress with a 10 year warranty. To me, it is very exciting, and I'm happy to share the following information.
We are planning on co-sleeping, and also needed a new mattress, so we didn't want one full of poison, but couldn't afford a latex mattress. We went to a mattress store in Cambridge, MA to talk to someone about alternatives, and the salesman turned us on to Gold Bond (Connecticut company) futon mattresses. I said, "Yeah, right, futon mattresses are very uncomfortable." His response was that the uncomfortable futon is becoming a thing of the past--that this company really makes a good one, and that futon mattresses don't have to be treated with flame-retardants (not all true: read on!!) We tried some and agreed they were comfortable, and decided to do more research.
Calling the Gold Bond company didn't help me, because the woman I spoke with didn't seem to know about flame retardants (duh!) so I decided to just go to a store (I went to Slumber Rest in Peabody on Rt. 1, which had AWESOME service.) They called the company for me and got the full scoop. Yes, futon mattresses have to be treated with flame retardants IF they include synthetic fibers like foam, but their cotton and wool models don't because they are NATURALLY FLAME RETARDANT--they'll just char, and not go up in flames. They sell them in Twin through King, and the one we got was Cotton with Coil springs in a Queen--with a 10 year warranty from the company. YAY!! When we got it home, it had a warning in it that said it was not flame retardant (also a warning to in general never sleep with an infant in bed with you--ha.) The cost of the Queen mattress was $250. That's what I call affordable.
I hope that you find the solution for you, but I'd certainly look into this option. We're really happy with our mattress.