Many hotels require this - the deluxe hotels that is. Disney has increased their valet parking fee. I don't think it cost extra to park your own car at Disney though.
I have been to hotels (city hotels mostly) where parking was $22 -25.00 per day!!! Even for self park!
Also some hotels charge "resort fees" (the Universal hotels do not, but the Swan and Dolphin at Disney charge an extra $10.00 per day) It's always a good idea to ask about extra charges when you book a room . And , be careful about phone calls - they can REALLY add up . It's best to take a prepaid calling card with you or your cell phone.