Yep I made some corner wedges of part cotton, part mineral wool. The cotton was a little tricky to cut--too tough for scissors, to stringy for electric knife--and it does compress quite a bit (I could have used just a little more material than the bales provided after compression), but I think it works well. I can't really do any measurements, but just guessing I would expect it to be able to absorb more above 1k than the denser mineral wool. Probably better at handling side reflections and other "grazing" reflections. Could be a little tricky to make it hold its form in a hanging absorption cloud though.
So to sum up, I think I would prefer 4" mineral wool for corner wedges; but if you got creative and used the cotton to shove into some sort of frame/holder, I think it can be effective as well. You could do some neat custom absorbers that would be difficult to do with mineral wool. As far as cost effectiveness, you gotta figure in the compression...