Medium firm bio foam mattress. One available at this price.

3/4 Blue Sky 100 Mattress

Start with a base of medium firm Soy Based Bio Foam then add a two inch layer of relaxing Resilient Foam and then top it off with center layer of cool gel memory foam.
These are topped with a quilted soft luxurious fabric cover that is filled with 20oz of soft fiber and one inch of Ultra Comfort Foam.
5 year non-prorated warranty if the mattress, when no one is on it, sinks more than 1.5". Optional years 6 through 10 warranty allows you to purchase a new Blue Sky mattress at half the current "on sale" price under those same warranty conditions (and you get to keep your old mattress)

Blue Sky 100 is very close to nine inches thick.