If you are definitely going to take the bus and not a cab, you could take the first park bus that comes and transfer to AK from there - you will minimize the wait time at your resort, but may have some wait time at the park you are transferring from.
Another option would be to hold out for the Animal Kingdom bus, since it will take you closest to your actual destination (and therefore least out of the way) and then transfer to AKL from there.
I do agree with byoung that you will probably want to be out there right at 7am to be sure you will be on time for your ADR. If you get to Animal Kingdom lodge early, I imagine it is a very interesting resort to explore in the morning.
My main concern on your behalf is that I have never done this exact trick and the busses for character breakfasts are said to start an hour before the first seating , while regular bus service doesn't start until 30 minutes before park opening - again, according to wdwinfo.com. This says to me that just because the busses are taking people to the theme park for character breakfasts, doesn't necessarily mean that they are picking people up at the parks until 30 minutes prior to park opening - does that make any sense or am I misinterpreting? It wouldn't be a problem if you were traveling at a peak time of year when the parks open early, early. However, if the park opens at 9 and the busses aren't picking up yet, it could be an issue for your ADR. Probably a question for the good people at the transportation forum, really!
Your safest, surest bet would be a cab, but if you do decide to try this bus trick that early in the morning, I would love to hear how it goes for you!
Sorry I have no guess on cost of a cab for you. I think it would be a good idea to be getting into the cab by 7:30, though. Better early than late to an ADR!
