Hi LightningBed,
As you probably know your budget is very limited and not enough to find the best quality materials in a mattress (depending to some degree on the size you are looking for and on whether you also need a foundation) so I would tend to focus primarily on what I call PPP (Posture and alignment, Pressure relief, and Personal preferences) more than the durability of the materials. Probably a good quality innerspring with reasonable quality polyfoam (1.5 lb or better) will be the best you can do but the bigger challenge may be finding a mattress that has enough padding and the right design to be comfortable for you. Some of the better quality and value options in your area I’m aware of are listed in post #5 here and you are fortunate that two of these are members here and would probably be the first places I would call and visit.
Phoenix