Hi KingSpore,
I’m not sure what size you are looking at but I would be aware that realistically your budget is too low for a reasonable quality memory foam mattress.
If you mean this one … it has 3" of 4 lb gel memory foam.
You can see my thoughts and comments on a solid surface support surface in post #10 here. It would be supportive enough but you could be increasing the risk of moisture retention in your mattress and the possibility of developing mold or mildew under your mattress.
Phoenix