Hi arisythila,
A “knockoff” could be anything from “cheap junk” to mattresses that use higher quality and more durable materials than Tempurpedic or anything in between. While I would certainly avoid the “cheap junk” (even if it “feels” the same as a Tempurpedic in the showroom or when it is relatively new) … there are many manufacturers that use the same or better quality materials as Tempurpedic that sell a “closely equivalent” mattress at substantially lower prices (in the range of half the cost of an equivalent Tempurpedic).
There is more about the different ways that one mattress can “match” or “approximate” another one in post #9 here.
Brand shopping can be among the worst ways to choose a mattress because you are buying a specific mattress not the brand and all manufacturers have access to the same or similar quality components and materials that they can use in their mattresses. Most of the more heavily advertised and most commonly available brands that you would recognize (such as Tempurpedic) are the ones I would completely avoid (see the guidelines here). The name of the manufacturer on the label also won’t tell you anything about whether a specific mattress is suitable for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) or whether there are any lower quality materials or weak links in the design. There is more about the risks of brand shopping in post #5 here and post #12 here).
There is also more about the most important parts of the “value” of a mattress purchase in post #13 here that can help you make more meaningful quality/value comparisons between mattresses.
Of course each person is always free to spend as much money as they wish to on a mattress regardless of whether their choice turns out to less suitable or less durable than another mattress at a significantly lower price but one of the goals of this site is to help the members here avoid some of the worst choices in the industry in terms of their quality, durability, or value and learn how and why to focus on the better ones.
There is also much more information about Tempurpedic in post #2 here and the posts it links to.
I would guess that there are probably better things you could find to spend an extra few thousand dollars on than Tempurpedic’s advertising costs and profit margins once you know why it’s completely unnecessary and how to find much better quality/value alternatives.
Phoenix