Hi NHN8IVE,
A mattress protector is different from a fitted sheet and is designed to protect the mattress from stains, body oils, and “accidents”. A fitted sheet won’t do the job of a protector.
There is more about the different choices and tradeoffs involved in post #89 here.
The least expensive of these tend to be the thinner “membrane” type but the higher quality ones should still be within your budget. If you want the tradeoff of the higher breathability and temperature regulation of natural fibers and are willing to replace waterproof with water resistance then wool would make a good choice but would probably be over your budget.
A stretch cotton protector may be a little less (although the better quality thicker ones are also fairly expensive) but you would be giving up water resistance for the sake of a nicer feel and less effect on the mattress.
There are none that have the benefits of all three that I’m aware of but for those that want or need waterproof (it only takes one larger accident with a pet to stain the mattress) then the better membrane types would probably be fine for most people … especially if budget was a primary concern. I would tend towards versions that used natural fabrics rather than synthetics though because they tend to be more breathable and cooler.
The pockets of the protectors are usually stretchy or fitted with elastic and will fit a wide range of mattress thicknesses and for the most part you should choose one that is at least several inches larger than the thickness of your mattress so that there is room to go under the mattress and its not so tight that it will come off or create a “drum effect”. They will normally list the size of mattress that they are designed for and if they don’t then the retailer will be able to tell you.
Phoenix