After so many responses, this had to have been covered, but I will say it as well.
Mattress's are HIGHLY negotiable(in mattress stores anyway). Notice how mattress stores always have a sale going on? It is commission based sales.
Also, all the mattress stores get their mattress's from the same places. However, each chain of stores renames each model so they can say they 'exclusively' carry X model of X brand. Look for the spring count and what the springs are made of. That way you can go to another store and compare prices. One store will have this fancy names model, but you won't be able to find it at another store. This is on purpose so it is very difficult to comparison shop.