Hi schneidy29,
As your post suggests you have already discovered … the most effective approach in buying a mattress is to first find the best sources and then to start with the process of choosing a mattress. The guidelines in post #10 here (and the information it links to) will greatly increase your odds of making the best possible choice.
Custom Sleep Design makes very good quality and value mattresses (and they are a little lower than the price you quoted for just a King “mattress only”) but they are also a more “premium” mattress and a purchase there would be somewhat like buying a custom fitted suit that was hand built to your individual measurements. They are definitely in a higher budget range than mattresses that don’t have the same degree of individual customization for each person on the mattress.
The better choices in the Minneapolis/St Paul area I’m aware of are listed in post #2 here.
You have some very good options available locally 
Phoenix