The passes break even point I think is eight days, If you get a good discount this could defray the cost of the Anual Pass (AP).
But if you do what we do and return back to Disney before they expire in a year, and stay for at least 4 or five days then you stayed for 2 vacations on one set of passes.
From Mousesavers.com
To reserve a "room only" reservation, call the Disney Reservations Center at (407) W-DISNEY and specifically ask to book through the Central Reservations Office (CRO). If possible, avoid booking your "room only" reservation through the Walt Disney Travel Company (WDTC), because "room only" reservations booked through the CRO system offer better terms for cancellation and payment dates. Here is an outline of the differences:
When booking a "room only" reservation through CRO, you are subject to very few restrictions:
You must pay one night's room rate as a deposit within 14 days of reserving your room.
Final payment of the remainder is due at check-in.
You can change or cancel your reservation without penalty up until 5 days prior to check-in.
When booking through WDTC, even a "room only" reservation is considered a "package" and is subject to the rules for packages:
You must pay a $200 deposit ($100 for Disney Club members) within 14 days of reserving your package.
Final payment of the entire remainder is due 45 days prior
http://www.mousesavers.com/disneyresorts.html#ap
use the above link hope it helps
Happy reading
Jack