Stay away from mattress stores - 200 - 400% markups are not uncommon. As mentioned, Amazon and the big box stores are your friends.
As for memory foam being warm, that may be true for a 100% memory foam mattress, but very few are made that way any more (Tempurpedic and a couple of others, maybe). We have a 10" full queen triple foam mattress we love. The bottom layer is memory foam (most of the thickness), then there are a couple of inches of support foam, topped by 1/2" (I think) of cool-gel foam. It's very comfy in both hot and cold weather and is GREAT for my bad back. Oh... and we paid less than $500 for it. Our son has had a similar experience with a similar mattress he purchased at Costco.
Rob
U.S. Army retired
2020 Solitude 310GK-R
MORryde IS, disc brakes, solar, DP windows
(Previously in a Reflection 337RLS)
2012 F350 CC DRW Lariat 6.7
Full-time since 8/2015