We just got our Blackstone 12" Memory Foam Mattress a few months ago at Costco. It was super affordable and we love it. Comes in a box that fit in our SUV and was easy to set up.
We purchased a Casper online about 7 years ago and at that time it was really affordable. We love the mattress. I recently saw that Costco is carrying them now. It is about 1000 for a King, which compared to some of the stores we check out is very reasonable. As I said, we've had the mattress for about 7 years and it's still like new.