There are annual dues, with vary depending on the resort. $450+. I believe you can pay these monthly only if you sign-up for automatic debit.
If you finance the purchase, there will be monthly payments until you've paid off the loan.
That's about it.
Of course there are other related costs, like airfare, passes to the parks, etc.
And if you use some of the other programs, like the Member Getaways, I believe there is a small charge ($75?) for each use.
Anyone else think of anything I'm missing?
Raidermatt