They are very expensive - in fact, my wife and I discovered it was cheaper to upgrade to the most expensive category in the most expensive monorail hotel than to rent a wheelchair van and stay in the moderate resort. In fact, the van would have cost more than our entire week long stay in the moderate resort.
I am not even sure if there are one-way wheelchair van rentals, so you may also want to look into train options (since I know flying with chairs is obnoxious)