This is gonna sound like some gay radio commercial, but I don't care.
I ALWAYS was the guy walking into the store telling them to give me the one for $100 bucks, and firm, if it comes in any range of firmness. I bought from Sam's for the wife and I, and Costco had the ones for the kids (since Sam's was about $40 more, and with 3 kids, an extra $40 means I coulda bought FOUR mattresses for the same price!).
My wife has nerve problems that are causing us to save to buy a REALLY good mattress. Select Comfort is what we'll be getting. I know (and DON'T care) that they are so expensive. If she's having a "bad back night", she can dial it up, and when she's feeling better, drop back down to something more comfortable for her.
If there's a place in the mall by you, go and test one out. You'll see it ISN'T the "that's just a stupid air mattress, it'll POP in a year!" concept my father-in-law seems to think it is, and as shmokes points out, factoring the cost out over the time you'll own it, it'll end up as chump change.
There's NOTHING wrong with going with a cheap mattress, but after testing this thing out EXTENSIVELY (to the point that the people in the store know us now and just say "hi, back to test it out again?" and then LEAVE US ALONE), we're convinced. If you knew just HOW cheap I am, that alone would be testament to how confident I am about that bed, but you're the one that has to convince yourself. Give 'em a try if they've got a store near you. You'll see what I mean.