It's interesting to examine this from the perspective of the number of onsite rooms in each resort category.
Touring Plans lists the number of rooms at each resort on property
here.
Category____Num of Rooms % of Total Rooms
Values-------10,308--------37%
Moderates----7,492---------27%
Deluxe--------5,652---------20%
Deluxe Villas---4,164--------15%
Total---------27,616
Assuming a consistent 90% occupancy rate across all resort categories for 365 nights, a $15/night resort fee produces the following revenue annually:
Values-------$50,792,670
Moderates---$36,916,830
Deluxe-------$27,850,230
Deluxe Villas--$20,518,110
Total-------$136,077,840
Because Values make up the majority of the rooms on property, Value resort guests would provide the majority of the revenue from a nightly resort fee.
The room counts for Deluxe Villas would be less of a factor in reality, since I expect the majority of those rooms would be filled by DVC reservations, not cash reservations subject to a resort fee. That would only exacerbate the proportion of total resort fees paid by Value resort guests.
Also, as previous posters have pointed out, a $15 resort fee is also a significantly higher percentage of the nightly cost of a Value resort room than it is for other, more expensive resort categories.
EDIT-formatting
