I am sure someone will chime in with the exact answer as I am not the expert. However like the buffet prices the dessert party is based on "seasons" so busy times/holiday's cost more. We just went in June and paid about $100 for the four of us.
I beleive that you should be able to book like 90 days out, but Disney doesn't seem to load the reservations as quickly as people would like and they have to wait for Disney to show it in their system in order to book. I beleive they are booking through October now, possibly a bit into November.
There are several Threads out here dedicated to just this topic and reviews as well.
Enjoy your vacation.