I bought a Linenspa mattress off Amazon on a whim for $125. It shipped rolled well enough to be transported in my sedan.
It is so comfortable, cheap, and convenient that I legitimately am stumped why anyone would ever spend more than $125 on a king mattress as a result. It's incredible.
Anyone spending hundreds or thousands on a mattress has been thoroughly duped and that's why there is such a war between mattress companies.