Hi bgilbert37,
The first place I would start your research is the mattress shopping tutorial here which has all the basic information, steps, and guidelines you will need to make the best possible choice … and know how and why to avoid the worst ones.
In low budget ranges such as yours it will be much more common to find lower quality materials in a mattress that may feel great at first but will soften or break down and can lose the comfort and support which was the reason you purchased it in the first place much too quickly (and the loss of comfort and support isn’t covered by a warranty … see post #174 here).
Post #13 here has more about the most important parts of the “value” of a mattress purchase that can help you make more meaningful comparisons between mattresses.
Post #4 here also includes some of the better online options I’m aware of that are in lower budget ranges and if you let me know your city or zip code I’d also be happy to let you know about any of the better options or possibilities I’m aware of in your area.
Phoenix