Air Mattres overlays

Not sure if anyone could help and will try to do a search on my own, but in general..How much do they cost? Does a rental fee of $1,100 a month sound right?? My father in law just got a bill for his sister who has been using one on her bed. This isn't a special machine or any thing.. He fought the price and they ended up telling him that he just needed to pay a one time fee to use this and that she MUST have this to prevent skin breakdown.. Aunt has pretty good bed mobility and this was initally used s/P AKA. I told him a regular matteress and a pillow under the stump would work just as well since skin break down isn't an issue..

TIA