Ikea in Canton, MI has some cheap memory foam mattresses, check the scratch and dent area for deals. If you have a Costco membership, they have some that are a bit more, but not $1000 (though they have plenty of expensive ones) But Costco's selection varies wildly. Walmart is worth checking.
A little farther out is the foam factory, at 17500 23 mile road. They have bare conventional, memory and latex foam. They also have covers to put over the foam. They have sample cushions of the different foams so you can test sit/lie down. That's what we did, then used a electric carving knife to round out the corners for our trillium
. We also just stuffed the foam in twin duvet covers we already had instead of buying a cover