Hi jackliao,
I would keep in mind that there are different types of foam including polyfoam, memory foam, and latex foam and there are thousands of different “foam” mattresses of different types that are available in the industry but the Tempurpedic mattresses use memory foam in the comfort layers and then most of them use polyfoam in the deeper support layers of the mattress (although some of their newer mattresses also use an air bladder or an innerspring for support)
As Jeff mentioned, Tempurpedic generally uses good quality materials but they are in a much higher budget range than many other memory foam mattresses made by smaller manufacturers that use the same or similar quality materials and I would tend to avoid them along with the other major brands as well (see the guidelines here).
I would also echo Jeff’s comments that price is often not an indication of quality or durability of a mattress in the mainstream mattress industry especially.
The first place I would start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choices … and know how and why to avoid the worst ones.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress that is the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for that are involved in each of them and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability, durability, and value.
If you are comfortable with an online purchase then the mattress shopping tutorial includes several links to the better online options I’m aware of (in the optional online step) and if you let me know your city or zip code I’d also be happy to let you know about the better local options or possibilities I’m aware of as well.
Phoenix