I agree that DIY memory foam mattresses are dirt cheap, easy, and last far longer than the prebuilt, but I'd actually advise against Bakari's suggestion of a dual 6" memory foam configuration. Not even Tempurpedic does that, and for a reason. One of the things that helps memory foam keep its shape and durability over the years is a firm conventional foam layer underneath the memory foam, and a solid flat surface under that. In addition to reducing the overall cost significantly, you won't actually notice any difference. I'm having trouble citing sources on this, but when I researched the topic myself a few years back, I discovered that there was a relative consensus indicated that there's no benefit on thickness above about 4" of >4.0 PCF high density memory foam, and the body cannot tell the difference. The important thing for longevity is the ability to rotate the foam, and have a decent cover. On the cover front, a fitted stretchy jersey sheet paired with a thin polyester batting (something loose and stretchy to add a bit of breathe-ability) should be plenty to deal with sweat between the foam and the regular sheets. I find that last bit crucial for muggy summer night sleeping. Also, the higher the density, the firmer the mattress and the longer the lifespan.
Not including the flat surface bedframe, you can actually build a Tempurpedic quality mattress for around $300 (depending on the size) if you shop around (Queen - about $100-200 for the 4" memory foam, $75-100 for the 4-6" high density poly foam, $20-30 for the polyester quilt batting and jersey sheet). Not much more price-wise than a very well-built, quality American-style futon mattress with coil springs new. Granted, you can restuff as explained already for far less, but I find the things a bit lumpy and although some can put a pricetag on a good night's sleep, it doesn't make sleep any less important.
I also learned from personal experience that the thinner toppers stacked together don't work too well. Splurge for the thicker solid sheets of the stuff. The homebrew memory foam mattress is still a chunk of change, but can be done in stages.