Research mattress prices both online and at reputable dealers. Compare mattress makes by their prices and features. Model names aren't always a reliable way to make comparisons. Sometimes dealers change the model names to confuse customers when shopping around.
Narrow down the choices and begin by making an offer on one, several hundred dollars lower than the sticker price. Begin negotiations from this point.
Remain uncertain that this particular bed mattress model is exactly what's desired. Flexibility is a must when negotiating. Silence and hesitation work in the buyers favor.
Settle on a final price and understand all that's included. Besides buying a mattress itself consider negotiating a box spring, bed frame, delivery and removal services in place of cost. If there's a warranty, understand those terms as well.