I don't know much about shopping for mattresses, but DH and I learned you can bargain. We went to Metro Mattress a few weeks ago after DD climbed out of her pnp. It was time for a big girl bed. The bed was was $450. We were very hesitant. The guy said it had been on sale the week before for $350 and there were still some left from that particular sale pack. So he called and got the ok to give us the sale price. We were still hesitant. Said we were going to think about it, look around and come back later. He asked where we wanted to be price wise and what price he could give us that would get us out the door then with it. We said we were looking more in the $200-$300 range. He made another call and got it for us at $250. It was painless. He wasn't super pushy and was really willing to work with us. It was great. We didn't even go in expecting to haggle about the price. We were simply hesitant because it was so expensive, wanted to call some family members, etc. He was eager to get us a better price.
We did not have the same experience w/ our king. We paid over a grand for it at Ashley Furniture, no bargaining. We'll probably shop around at the big stores next time and end at Metro with an idea of what the competition is offering.
So, long story short, (too late), don't be afraid to haggle or just appear hesitant so they offer a better deal.