Check out Mousesavers.com - it has a full analysis of when it's economical to purchase an AP pass, and the historical rates you're looking for. Happy Planning!
There is also a good account of what kind of discounts to expect. keep in mind that AP rates usually aren't announced until a few months out. So, if you have definate dates, it would be wise to book now and hunt for discounts later (if they are offered). Past discounts have been anywhere from 20-40% off rack rates, depending on category of resort.
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