How much to add it on depends on how you booked/who you booked through.
For the Quick service dining plan you get 2 quick service meals (Entree or Combo meal, dessert & soft drink), 2 snacks per person per night & a refillable mug per person per reservation.
We are trying on our trip this time as we didn't want to be tied to reservations with DD.
This is the official leaflet for the next one up, the standard Dining Plan, we have this free with our package this August, cost if you book with Disney.com $40pppn. If booked with Disney.co.uk it was £20 but I think they increased it to £25
Each day we get 1 counter service meal, 1 Table service meal and 1 snack. http://disneyworld.disney.go.com/me...09 Disney Dining Plan - Walt Disney World.pdf
This was the reason why we chose the QSDP - we've paid £15.99 per adult per day - which with the current $ rate works out about $7 les than the $29.99 they are charging in the US for the plan! Booked with Disney UK. As someone has already said, the price varies depending on who your package is booked with.
Some will say you get more 'value' from the DDP (that includes 1 TS, 1 QS 1 Snack) but you need to decide if you want to be tied to a daily ADR??