Hi Buttercupbetty,
Just to clarify … soy foams and other plant based foams are polyurethane foams. You can read more about it in post #2 here.
The guidelines here and the post about durability here both include the minimum density polyfoam I would consider.
I would also agree with Blake that flipping and rotating a two sided mattress more frequently at first and then once a season after that will extend its useful life. This is one of the advantages of a two sided mattress.
1.8 lb polyfoam is a good quality material … especially in a two sided mattress in the budget range you are looking at.
Phoenix