what product do you use 6+ hours every day of your life for the next 10 years? if anything justifies the cost, it's a matress
that said, companies know this logic and feel they can charge a metric fuckton for them now. about 8 years ago, we got one of those mattresses that one person can sleep on and the other can do jumping jacks. my wife moves a ton in her sleep, so we wanted something that would help me sleep though. it doesn't prevent her from pulling the covers enough to roll me over, so it wasn't a complete fix.
give it's age, it's no longer flat. there's 2 dips where you can see we've been sleeping for years. i'm of the opinion that it might be better to get the cheapest memory foam style mattress w/ the intent to replace it every 5 years or so, instead of a more expensive model that you hope to keep for 20.