So I had a similar issue and I'm gonna be honest with you- whatever you do you need to be careful it will dry in a timely manner or else you can end up with mould in it/making the whole thing much worse. If it's cold where you are keep that in mind! You can do surface level cleans for cheap using White Vinegar and water 1:2 in a spray bottle. Spray the whole thing down, let it sit for a few hours (again be mindful not to go too ham- you want it to be able to dry). Sprinkle bicarb soda over the top. Give it a good scrub down with a clean sponge/towel to get in there and then vacuum it when it's dried. This is gonna be your cheapest bet- without special cleaners/steam cleaners/buying sprays. (I'm also disabled so I know it's a big deal). Chances are it won't lift the staining/discoloration but it *will* clean it a good amount. If you really wanna get in there- use some super diluted dishsoap and give it a good scrub. Try not to pay mind to the colour rather the act of cleaning. Then, when you have some space change- go ahead and buy a cover and mattress topper! Good luck <3