Hi rellwood,
It could be both as well as between the top and bottom of a Dunlop topper (see post #6 here).
All foam materials will go through an initial break in period where they will lose any “false firmness” in the material before softening more slowly over a longer period of time. Both the initial softening and long term softening would be less with higher quality and more durable materials but the firmness of the material and the weight and body type of the person using it will also make a difference.
All foam materials will also soften temporarily with use overnight to some degree and with higher humidity conditions or warmer temperatures inside the matttress the foam can take a little longer to recover (Dunlop more than Talalay).
Yes … this is the reason for the lower price but there would be little difference in using them as the top layer of a mattress or as a topper.
If you are looking for a cover that surrounds the topper to protect it from breaking down as quickly with exposure to ozone and ultraviolet light (which are some of the things that can degrade latex prematurely along with the others listed here) then mattresses.net sells covers and there are some other sources for covers listed in post #4 here which can help protect the topper from substances that can degrade it and from damage from tearing as well. Covers that are more stretchy will have less effect on the “feel” of the latex than a woven cover but they may be more costly.
Phoenix