To adjust any reservations already made (resort reservations that is), you're going to have to call. Prices for
DDP for adults is:
Regular Dining Plan (Non-Peak)
$51.54 per night ages 10 and up
$15.02 per night age 3 to 9
Regular Dining Plan (Peak Season)
$53.54 per night ages 10 and up
$16.02 per night ages 3 to 9
Depending on how you booked your reservation, will determine what affect adding the dining plan has...
- If you booked your package as a package including tickets, and you are more than 45 days before check-in, nothing will change other than the price.
- If you booked your package as a package including tickets and you are less than 45 days before check-in, you will need to pay the difference in full
- If you booked your reservation as room-only, you will have to convert to a package and follow the package rules (45 days out payment due, etc)
- If you booked cash room-only without tickets, you will also have to add at least 1 day tickets for each member of your party as well as adding the package rules
- If you booked AP room-only (either rack rate or AP discount), you may add the plan as part of a "ticketless" package. Again, standard package rules will then apply
- If you rented (or are using) DVC points, you may add the plan without adding tickets, but payment is due in full (no matter how far away you are)
- I believe that some military packages can also qualify for the "ticketless package", but I'm not 100% on that
Phew, that was all from memory ><
For all package changes, at less than 45 days out, you may be charged a small fee for modifications, this is typically waived if you are increasing the total cost. At less than 10-14 days out, this fee increases and is typically NOT waived, no matter the direction you go. At less than 3 days out, you cannot add, remove, or modify dining plans associated with your package if you booked anything but via DVC points. At less than 2 days out, same deal as the previous line, but only for those staying on DVC points.
I think I covered everything

(ETA: Except the prices... you know, the actual question...)