Amazon has Home Life Comfort Sleep 6-Inch Mattress - King for $82.37 ( Lowest Price ever according to Camel). Shipping is free.
I combined this with the below $54 Bed frame from walmart :
Mattress comes compressed and rolled for easy shipping
Fits any Standard King Bed Frame
Quality foam and polyester layering between cover and coils for optimal comfort
Independently superbly pocketed 15-gauge pocket coils
More comfortable than most standard 8-10 inch mattresses. Plus No squeeking!