Hi Kristen,
There is more about some of the factors that can affect the price of a mattress relative to different manufacturers or retailers in post #14 here but two of the biggest factors are the type, amount, and quality of the materials inside it and the size of the mattress as well (bigger sizes are more costly than smaller sizes). Having said that … you certainly don’t need to spend $4000 to purchase a high quality mattress (see this article) and there are many high quality mattresses that are available at substantially less than that.
The better options or possibilities I’m aware of in the area around Minneapolis/St Paul (subject to the quality/value guidelines I linked in my last reply) are listed in post #2 here.
Phoenix