Isotonic memory foam responds to body temperature and is said to fit and "remember" the sleeper's body even better than regular memory foam because of it. That is why when shopping for a memory foam mattress top in the 3 inch memory foam or 4 inch memory foam variety, isotonic is the way to go. Different companies charge different prices for isotonic pads, otherwise known as memory foam overlays, but generally the prices range from $300-$899.