damn sure a $6000 mattress will be better quality and won't sag on you in 3 years. Your $200 mattress might be comfortable at first, but it won't last. The reason mattresses cost more is because of the materials they are built with.
Lay down on every mattress you can find and buy the one that's comfortable to you. Don't pay any attention to brand and don't pay attention to price (as long as it's in your budget). There's nothing saying a $6k mattress will be more comfortable to you than a $200 one.