There is a huge mark-up on mattresses (furniture in general)...and mistresses too if they are firm and feel good.
I've never bought a mattress w/out going to at least a couple of stores and getting a few hundred dollars off...."because that's what the other store is willing to do." I think I got $400 off a Sealy king set a year ago.
Sealy is the best mattress I've ever bought. As a matter of fact, I bought a $350 Sealy queen mattress when I graduated from college and started to sleep on something that didn't have an F U and a TON in its name.
That mattress was great for my back. When I moved, I gave it to my parents and they loved it....and are still using it. It's been 0ver 10 years.
So you can get a good mattress for $600 or less.