In the past 5 years I have owned a Casper at $850, a tuft and needle for $750, and something by Ashley for $350. I think I like the Ashley slightly better but they are virtually indistinguishable. I second the recommendation to go cheap.
I've had a lot of negative things to say about Amazon for the past couple of years, but yeah, you shouldn't be paying much more than $300 for a mattress. Everything on top of that is your customer acquisition cost, which should be in your bank account, not Google/Facebook's.
In my experience you can just optimize the firmness to your liking with a pad. If suddenly its too firm or too soft, you can switch it out and not have to replace a whole mattress. Relatives visited and wet the bed? Throw it out and get a new one, your only out a few hundred dollars instead of $1000+.
For couples who want their mattress different ways, that is another market.
