Have the Costco foam king mattress on a wood platform bed I built out of excess plywood and 4x4"s. It's great. I used to have lower back problems, even though I get up and walk around almost all day at work. This helped significantly. My father, who has a lumbar spine injury, also uses it.
It may be a little more expensive, but consider also sticking two or three 2" thick mattress topper foams, as the full thickness may be too much if you want a very firm mattress.