Hi sneezealiday,
As you can see in post #2 here and post #2 here … I personally wouldn’t deal with FBM for any material where the accuracy of the specs or description was important.
Memory foam is not suitable for use as a support layer. Memory foam comfort layers need a firmer support layer underneath them which is not viscoelastic.
$80 will buy you a cheap one … not a nice one (or at least not great quality). Good quality mattress covers can be much more costly than this. You can see some sources for mattress covers in post #4 here.
The thickness of a mattress really has little relevance by itself (some people would do very well with a 6" mattress in some designs for example). If the mattress provides you with the PPP you need in all your sleeping positions (Posture and alignment, Pressure relief, and Personal preferences) then it’s “thick enough” for you.
Support layers that are thinner will tend to be firmer and less “adaptive” to different weights and sleeping positions than thicker support layers using the same material in the same IFD. They have less “range” or “room” to go from initial compression (when they are softest) to fully compressed (when they are firmest).
Phoenix