R.D.
We just bought a serta perfect sleeper, king size, at Mattress Warehouse for $800 with the boxspring. After pricing around, that was the best mattress for the price. It's a $1300 mattress on sale, WOO HOO! I'm loving it. I haven't woken up with any back pain since we got it :)
The downside... this mattress has a 'life' of about 5-10 years, which is on the low end (most last 10-20 years)... BUT the good thing is, it has a 10 year warranty... if we are sinking more than and inch into it, they'll replace it for free :)