When are there diminishing returns?

Hi, so I'm currently sleeping on a full size green foam + box spring that was my sister's and was then orginally bought from an estate sale so it could be infinite years old.

I've been going through a ton of mattress reviews and read through mattress underground and everything but I don't know how much money is a realistic spend. It's so confusing that I'm really close to just buying a latex topper for my current mattress so I don't have to deal with it.

I have been reccomended to buy from a local store called the orginal mattress factory but I will also probably go to a mattress firm to try the online ones. I sleep on my back but with my head turned to the side (this is probably fucking my neck up ngl). How much money *should* I be budgeting for a mattress that is better than my current.

Do I need to replace the box spring? Or can I just buy a new mattress and use the old one? I'm seeing mixed stuff online for that a box spring goes bad before a mattress does to that I don't even need a box spring if I have a slat frame.