Hi mattressseeker,
You will usually pay more for a mattress at stores that play the game of “fake sales” or “negotiable prices” because they just inflate their prices and then “allow” their customers to “negotiate” part of the higher markup back again so they will “feel good” about paying higher prices and the “deal” they think they received. Better stores (like Jamestown) sell their mattresses at their best price every day of the year and rarely have “sales” and when they do they are for smaller discounts. There is more about “negotiated prices” in post #6 here.
Of course it never hurts to ask “is that your best price?” 
Phoenix