We rented one for my mom a couple of years ago and it was $8 for the day I believe. They also have ECV rentals but those are quite a bit more ($35/day or so) - the rates have probably increased, like everything else, but that at least gives you an idea. The process was extremely easy. We just went to where they rent out strollers at the front of the park (we were at Epcot that day). My dad rented it so I'm not sure what kind of info he had to give, if any. It's not like you can steal them, since they won't let you leave the park with their equipment.
hth!
Cheryl