IKEA for example sells a lot of mattresses under 150$. Under that you can just by foam pads, but the biggest savings is simply avoiding mattress stores or companies.
If you like pillow tops or those egg shell foam pads you can also just buy them separately.
One interesting diy hack I read about is to put insulation or sound proofing around your dishwasher. Basically $20 in insulation can turn an old dishwasher or a $200 dishwasher into something super quiet.
I've slept on a 150 dollar Ikea for years and I've slept on a leesa. Maybe these conspiracy theories are true, but the Leesa is much more comfortable than the cheap Ikea. Ikea also has more expensive substantial mattress options, but it looks like they approach the same price range anyway. So what's your real suggestion then? Sleep on the floor in a sleeping bag?
Personal preference does not make any option objectively better. So, my suggestion is to explore your options. I enjoy a 4” foam pad on plywood over several thousand dollar mattresses, but it’s not a question of cost just comfort. Water beds are another cheap option that have largely fallen out of favor.
PS: Body weight is a huge part of this. Gain or lose significant weight and your preference is likely to change.
If you like pillow tops or those egg shell foam pads you can also just buy them separately.