I switched both of your new threads to the thread you had already started since your questions have already been answered there and are becoming redundant.
If you are now more interested in doing some research rather than just having someone tell you what to buy (instead of how to choose) then the first place to start is post #1 here.
I know you want to get by on the cheap, as it were - but as my Mama always told me, sometimes you get what you pay for. Personally, I’ve never seen a $20 cover for a latex mattress that I’d buy, because paying that little for the cover would tell me it wouldn’t be all that long before I’d be paying someone else for a better cover. From a financial point of view, I’d far rather buy one good cover than two or more cheap ones that turned out not to be worth even $20.