Hi ajsb728,
Just in case you haven’t read it yet … the first place I would start your research is post #1 here which has all the basic information, steps, and guidelines you will need to make the best possible choices.
A mattress is only as good as its construction and the quality of the materials inside it regardless of which manufacturers name is on the label. Coaster is a furniture distributor and importer and King Koil licensee and the quality/value of their mattresses would depend on the specifics of their design and most importantly the type and quality of the materials in the mattress. You can see the criteria I would use for memory foam mattresses in post #10 here. Once you know the specifics of what is in a mattress then you can make meaningful assessments and quality/value comparisons with other mattresses. If you can find out the specifications of a mattress you are looking and post it here I’d certainly be happy to make some comments about it.
There are several mattresses that are called “mattress in a box” so again it would depend on which specific mattress you were looking at and the specifics of the materials in the mattress. Do you have a link to the mattress you are looking at or the details of the layers?
Phoenix