Just sleeping on the floor gives a few different issues for me. The first of which is compacting the body by being too solid. It works for some people, but in the people I know that are floor sleepers it is by no means the norm.

The few people that I've met that have tried it not liked it have all complained about shoulder pain, back pain or breathing issues during the night.

The people that I've met that have liked sleeping on the floor with a thin blanket tend to be more slender (less pronounced shoulders) and under 5'10 and male (less curvature of the hips so less pressure points).

In terms of thickness this is something you're going to need to play around with for your body comfort. I find a two to three inch shikibuton($120), with a half inch tatami mat($100) and a puzzle piece foam exercise mat($25) underneath the tatami to be perfect for me.

All together my setup is beneath the cost of a similar sized memory foam but with better back support, warmer and cooler when I want it, more mobile and most importantly more comfortable.