Usually the more expensive mattresses are the ones near the front. The cheap mattresses usually aren't even set up on a bedframe, but are stacked on their side toward the back of the store.
Those are usually the ones that are advertised as on sale. You should make sure that you bring any advertisement with you to the store, because otherwise they will go based upon the prices on the tag and pretend that knocking a little bit off that price is a big deal. It's usually nowhere near the advertised teaser deals. It's also good, if possible, to go in with a friend or relative who has a military id, or who is a senior citizen, since they will sometimes give discounts to those individuals.
Another game the mattress stores like to play is charging for the mattress frame and delivery. In reality, you could probably find a perfectly fine, basic mattress frame for $5 to $10. They will charge a minimum of $40 to $80 for essentially the same basic metal frame. This doesn't include any special head board, foot board, etc.
They will usually also charge at least $40-60 for same day delivery. If you have a friend with a truck or van then you can save on this too.
Often the stores will claim that they have money-back guarantees, but the major conditions is that there can be no stains, snags, tears, etc on the mattress and the legal tag cannot be removed. I would recommend that you buy a can of Scotchgard and spray the mattress thoroughly. This will make it much easier to remove stains that may occur. Most people don't want to sleep on a plastic protector sheet that they sometimes make for people who are chronic bed-wetters, though I've known people who will take these kinds of measures to protect their beds.
Many of the mattresses come with 5 or 10 year warranties. Of course they do not cover all types of damage. Usually they cover defects that might lead to "collapse" or "mattress failure". These warranties are usually factored into the price, but I would not buy any additional warranty. In some of the places I went they were selling warranties as long as 20 years, but realistically, who is going to have the same mattress for 20 years? Do you really know that the company you bought it from will be there in 20 years to honor the warranty. 10 years is the max that would probably be reasonable for most people to warranty.