Hi Lasdj,
Most of the major brands that use the same technology use very similar membranes. The differences in prices besides the natural differences between brands and sources is often about the quality of the material that is bonded to the waterproof/breathable membrane and whether it has material on both sides. You can get these in versions which surround the mattress or in versions which go on top and tuck under the mattress like a fitted sheet (which would let a little more air into the mattress which would be my preference unless of course protection from allergens like bedbugs or dust mites is important)
There are links in post #4 of this thread to a few examples of similar brands and to some of the tradeoffs involved in your choices. Most of the newer models using good material and a quality membrane aren’t noisy like the old vinyl protectors (like the ones that go on children’s beds) which are more waterproof but not breathable and tend to sleep hot.
If you are looking at various options that all seem to have similar features (a breathable/waterproof membrane and similar quality material), then I would make my choice based on price.
Phoenix