Hi JLC777,
I think that one of the reasons that testing a mattress in person is that each person can have a very different “rating” for any mattress. What feels “plush” to one person can feel firm" for another and of course vice versa as well. these types of mattress “ratings” really are relative to each person.
It’s the same with a topper being added to a mattress as well. One person’s firm is another persons soft/plush. Testing a mattress topper combination in person or at least a mattress that has a similar layering inside the mattress as you are considering in your combination can provide a very useful reference point when you are buying a topper online. The construction and design of your current mattress will also play a big role in the type and thickness of topper that may work best for you. I would be very cautious adding a topper that may be too thick and soft (in combination with the softer layers that are already in your mattress) to provide the support and alignment that is just as important as pressure relief.
There are some guidelines that you can use in choosing a topper in post #8 here and some of the better sources for an online topper are listed in post #4 here which includes several latex options that are within your budget although 100% natural talalay may put you over your budget a bit. I would also keep in mind that in the softer ILD’s that 100% natural latex may not be as durable as blended Talalay or 100% natural Dunlop.
Phoenix