Sara,
Yes, there are in-park breakfast buffets at each park. I list them below, by park, with what characters are at the different meals. As for getting in early, if you have reservations for a time prior to opening, you will find they have an attendant at the park gates that will check you in and direct you to the restaurant. You can't start riding anything, since they are not open, but walking the parks when empty is pretty cool, and a great photo opportunity.
As for cost, none of them are down at the $15 per person level. For three of the four you would pay in the $25-30 per adult range, and $15-20 per child. This estimate includes tax, but not tips. Also you should know that at certain times of year (holidays, for instance) the cost will be higher. You have to ask when you make the reservation to budget correctly. The fourth, the one at Epcot, costs more in the $40 per adult/$25 per child range, and also going up for holidays.
Here is what is available, by park;
- Magic Kingdom: Crystal Palace with Winnie the Pooh and friends
- Epcot: Akershus Royal Banquet Hall with Princesses
- Hollywood Studios: Hollywood and Vine with characters from Disney Junior Channel
- Animal Kingdom: Donald's Safari Breakfast with Mickey and friends
There are a few resort based buffet breakfasts too, more of these with Mickey Mouse and friends. These are not significantly lower in cost, and can be higher in some cases. One that might be a good choice is over at 'Ohana, where Lilo, Stitch, Mickey, and Pluto visit, and the cost is more in the low $20s for adults, and low teens for children.
Write me back if I can help more!
Have a magical vacation!