Hi abe,
Yes … soy foam (or any of the other so called "plant based foam’) is mostly a marketing term and as much about greenwashing as anything else. You can read more about these types of foam (which are still just polyurethane foam) in post #2 here.
You can read a bit more about the Evosleep Blossom in post #10 here. It only has an inch of latex and 1.8 lb polyfoam in the support layers. I would want to know the density and thickness of the polyfoam in the comfort layers because this would likely be the weak link of a mattress like this.
A mattress is only as good as the quality of the materials and components inside it and before making any mattress purchase I would want to know the detailed specs of all the layers in the mattress (including the thickness of the layers, density of any memory foam or polyfoam, and the type and blend of any latex).
The “value” of a mattress is not relative to it’s regular selling price so much as it is in how it compares to other similar mattresses. I probably wouldn’t buy a used mattress at all when there are so many good value new choices but If I was to consider one I would be looking at a maximum of 50% of the cost of other similar mattresses (and possibly even less that that of its original price if the original price was already more than other similar mattresses).
Phoenix