My good king mattress cost me over 3k on sale with an adjustable base. That's a "luxury" mattress that's top of the line for a traditional mattress but keep in mind that for 3k you still won't even be touching something like a tempurpedic king.

My guest bedroom has my old mattress which is my old one that I only kept for about 2 years is a ~$1300 queen. Before that I had a $400 queen. I can honestly say with my back problems that I definitely saw a thousand dollar increase in quality of life on the first jump, from queen to queen, but when I went to luxury king with adjustable base from queen I did not see a $2000 increase. It's definitely better and it's honestly amazing but is it $2000 more amazing than the old queen? No not really.

My opinion may be different if I'd been able to get a tempurpedic style mattress but my wife was fully against it, she can't deal with them for whatever reason.

Would I do it again? Yes, definitely. Each step has helped my back further. Is it a linear increase? No, most of the improvement you'll get will be when you go from a cheap sub $500 mattress to a $1-1500 mattress.