Since we liveaboard, we splurged and got a Purple mattress. It was around $900. According to our chiropractor, who has one and loves it, she said many other chiros she knows have opted for the Purple mattress as well.
It came tightly rolled, so it wasn't too hard to fit down the aft stairs (though it does weight quite a bit!). We also have rounded corners at the foot of the bed and were going to hot knife it to fit, but as someone else mentioned the corners now just hang out/over the end like a regular bed. We don't even notice. We do notice that we are getting much better sleep, though.