You pay a thousand bux for a memory foam mattress. "Memory" means it takes the shape of your body and holds the heat in. It feels like you are encased in wood so you can't roll even a little bit.
A traditional spring mattress, if it's a good one, is quite hard. I inherited my mother's mattress and after a short time I went looking for something else.
The very best mattress is a waterbed. Unfortunately, every bed peddler in the country has invented something called "waterbed" that only they sell, so it is somewhat difficult to find a plain waterbed. Walmart and Amazon sell the vinyl bag for about 50 bux, but then you have to build a frame according to your own cleverness. And of course a waterbed always eventually springs a leak. And of course you can not have a cat at all.
The second best mattress is an air mattress. Walmart sells the Beautyrest at 60 bux. An air mattress does not last forever, but it is cheap enough to get two at a time so you have a spare. If it lasts a year your average cost is five bux a month. Any other kind has to last ten or twenty years to get your average that low. You have to have an electric blanket under the air mattress, 50 bux or so, and you have to have a terrarium thermostat, 20 bux at Walmart or Petsmart or any local pet store, and you have to find some way to tie the sheets to the mattress. Get bra hooks and stretch tape at any sewing supply store and tie the sheets to the electric blanket.
Now you are sleeping in heaven!