They are just a standard play yard, like the Graco Pack 'n' Play (I don't know if it's actually Graco brand, but it's exactly the same). There is no charge for it, but you should request it on your reservation, to help ensure they will have one available when you arrive. In our experience, it was clean and well-maintained.
We've used it once when my son was 6 months old, and we will request one again for when we travel in February with a 9 month old. Not Disney, but my son still sleeps in his when we travel to the cottage in the summer, as recently as this past summer when he was 2 years old.
We brought our own PnP sheet just in case, but housekeeping brought us a couple Mickey sheets with the PnP, so we didn't need to bring our own.