The new Tulo Liv mattress has a pretty solid sale right now. It’s $299 for a queen mattress.
From my research it’s been on sale below $500 before, but I don’t think it’s been below to $300 before. There’s more discussion in the foam mattress thread.
Also, their original Comfort mattress is $499 instead of $699. This is the one that the Wirecutter reviewed.