It is $4.00/adult and $2.00/child.
As Nala said, Disney doesn't advertise it, but the upcharge/surcharge prices have been loaded into the computers for months now, so when you call Dining and ask for summer prices (holiday has been quoted to me as being June - mid August this year), they will give you them with the upcharges included. Disney doesn't advertise, and this is sneaky, IMHO. But I can assure you that the peak season upcharges are in full effect for most of this summer.
Also, for any pay in advance meals, such as CRT, you can see the actual upcharge/surcharge online. If you do 2 different ADRS, one for peak summer season, and one for say Sept or Oct., with no peak season, you can then compare those prices. When you make an ADR, you will get the itemized cost per person, and so right before you have to confirm with payment, you'll see these costs on the ADR summary page.
HTH, Tiger
