Mattress-buying has got to be one of the most hassle-filled transactions that a consumer can go through nowadays. Manufacturers are not open with information regarding their products, and due to the vast amount of differentiation in the market, it is almost impossible to do an accurate price comparison. Thankfully, the market is also highly competitive, and as everyone here as said, you can often get a decent deal.
A few warnings, however... Innerspring mattresses, ie. the most common mattress on the market, have an extremely high failure percentage, estimated around 40% within the first year. That is, 40% of innerspring mattresses will have noticable sagging or other failure within 1 year of use. Compare this to the often enormous price tag that comes with the high-end innersprings, like Kingsdown or Simmons, and this is a terrible deal for the consumer. Factory warranties, touted as a good deal on the front end, seem to be almost impossible to fulfill.
The last year, I went through 3 innerspring mattresses due to this problem, one, a Simmons Beautyrest, and the other two, Sealy mattresses. I'm not a fatty, I only weigh about 175 lbs. It's just the crappy construction.
I'm also a grad student, and pretty poor, so I eventually just went to IKEA, and got one of their mid-level foam mattresses. I've been FAR happier with this than anything else, and I saved about $500 compared to what I spent on mattresses so far. I'm not recommending IKEA at all, but I think you should consider looking at what you can afford in latex or foam before you spend some big bucks on an innerspring.
Here's a good website that provides some data on the various brands/types/makes/etc...
http://www.sleeplikethedead.com/bed-...view-home.html