We have been sleeping the Wal-Mart Ozark Trail air mattress for quite a while now. The flocked tops are hard to patch, but the vinyl side (and the non-flocked ones) are much easier to patch.
We are currently sleeping on an all-vinyl ($15 queen) bed. I have an "Unfeather Bed" pad (from www.domestications.com)
and a heated mattress pad because we are used to sleeping on a heated waterbed. David & I also have bad backs... we air up the mattress until fairly firm (test with both of you in the bed as it does make a difference). We do periodically have to add more air. I think the vinyl stretches slightly over time. I bought the electric pump so airing up is pretty easy.
I can buy an awful lot of the Ozark Trail mattresses for what one Sleep Number (or most any other fancy air mattress) will cost.