Hi mjw,
There is more about the different ways that one mattress can “match” or “approximate” another one in post #9 here. Any mattress that uses the same or better quality/density materials would be very similar in terms of durability. The tutorial post also includes a link to a list of some of the better online memory foam manufacturers/retailers that I’m aware of as well (in the optional online step) and many of them also make memory foam mattresses that are designed to compare well to many of the Tempurpedic mattresses either in terms of firmness and feel or in terms of the quality/durability of the materials (or both) that are in much lower budget ranges than the Tempurpedic mattresses that are equivalent to theirs.
There may also be other memory foam mattresses available locally that compare well to the Tempurpedic line as well either in terms of their firmness and how they feel or the quality of their materials (or both) but you would need to test them in person to compare how they feel and you would need to know the type and quality/density of the materials inside them to know how they would compare in terms of durability (see this article)
There is also more about the most important parts of the “value” of a mattress purchase that can help you make more meaningful quality/value comparisons between mattresses in post #13 here.
Phoenix