Bottom line on mattresses, and I know this because my former stepfather owned one of the major bed store chains in the Hudson Valley... Your $6000 mattress probably cost the retailer $2000. The guy was loaded, even after dropping god knows how much on an 80's coke habit that would make even Tony Montana blush.
If you simply asked, you got free delivery, free box spring, free mattress cover, free pillows, etc. If you were savvy enough to barter with a jew, you could get him down 20-30% on top of that.
Buying a mattress is very much akin to buying a new car. The markup is massive, and there is always room for serious negotiating.