Mears is probably your cheapest option, at $10 per person, roundtrip. (You pay Busch Gardens when you get there, so my guess is that the trip is subsidized by Busch itself)
https://secure.mearstransportation.com/MearsExpress/BuschGardens.asp
The hitch is that you have to get yourself to one of the pickup locations. The closest one to the All-Stars is the Best Western Lakeside in Kissimmee, which is on Route 192 just west of Disney property. Taxi fare (which covers everyone in the cab, not per-person) for that shouldn't be more than $10-$15 each way, plus tip.
A suggestion: Tell the taxi driver to go toward Animal Kingdom Lodge and take Sherberth Road. It's *much* faster and avoids all the traffic and signal lights you get going via World Drive and 192. Barely a 5-minute drive.
Oh, and if you read the additional info on the Mears page, the bus fare is included in some park ticket options.
EDIT: And another hitch is that 9:40am seems to be the only available pickup option there (other locations are 9-9:30, but are farther away from Tampa). Drive time is prob a little more than an hour, so you're arriving at 11am (traffic willing).
So if you want to get to the park at opening, the bus is not for you. Your options include a private towncar or taxi, both of which are VERY expensive, or renting a car for the day.
-Rob