$500 - are you kidding?? You get what you pay for...
You've got to spend more than that for quality. We spent $4,000 on a Tempurpedic and it's worth every penny 3 years later. I can't imagine investing less that $2,000 for something as important as your bed. Your comfort, health, and well-being are at stake.
If you're limited on funds, try Costco. They often have excellent mattresses at a fraction of the retail price.