Hi coppercreek,
If you have the Tempurpedic Original then it would have 2.8" of their firmer 5.3 lb memory foam over a 5" polyfoam base.
If you have the Tempurpedic Classic then it would have the same 2.8" of their 5.3 lb memory foam and another .8" of a softer version of the same density memory foam over this and the same 5" polyfoam base layer.
Both of these have been discontinued and the closest they currently have would be the Contour Select which is currently the firmest mattress they make and has 1.2" of the softer 5.3 lb memory foam over 2.8" of the firmer 5.3 lb memory foam over a 6" polyfoam base layer. They rate this as “extra firm” but it’s softer than the original was. You could test this in any store that carried Tempurpedic to see how you think it compares to your mattress.
The Tempurpedic memory foam is firmer, more temperature sensitive, and slower responding than most of the current memory foam on the market but the closest you would likely come in “general” terms would be about 3" of 5 lb memory foam over a 5" or 6" firm polyfoam base layer (assuming that you have the Original 8" mattress) but this would depend to some degree on the specifics of the memory foam that was in the mattress because there are many different types of memory foam even of the same density.
Your budget is very low for a mattress like this and most memory foam mattresses in your budget range would use lower density foams (the Tempurpedic Contour Select is now selling for $2049 in full size).
Of course there are many options that use similar or better quality/density foams and sell for much less than the Tempurpedic and some of the better memory foam manufacturers that sell online are listed in post #12 here and would be well worth talking to to ask them how their mattresses would compare to the Contour Select or which one that was in your budget range may come the closest. If you let me know your city or zip there may be some local options available to you as well that you could test in person to see how close they felt to your mattress.
Phoenix