No usually and they are normally in good locations on the ships for accessibilty to the lifts etc, and they ae much bigger than the normal cabins and no extra charge but get in early as they tend to book up early. The ships these days have a lot of people on board with disability and wheel chairs and they get about all over the ship with no problems. The only problem is not to pick a crusie with tendering if possible as the ships companies will not allow you off the ship at these ports as they will not accept the liablity and it is sometime not easy for able bodied people if the weather is a bit rough or there is a swell. All the best
I was fortunate enough to share a disabled cabin with my disabled brother last year on the Thomson Celebration and there was no extra charge for using this cabin.
I don't know if any other cruise companies charge but you could look on the internet or enquire with these companies as to whether they do charge or not.
No, no extra charge otherwise they would be sued for discrimination.