From what I have heard from friends, the Maryland Transit game day bus is very unreliable.
What I would recommend doing is taking the Green Line train to Greenbelt and then switching to the B30 BWI Airport bus. After it drops off at the airport, it stops at the BWI light rail station. Hop on light rail and take it to the Camden Yards stop.
Your o/w cost per person is:
MetroRail Fare: varies based on where you board ~$2-3
B30 MetroBus Fare: $6
Baltimore Light Rail: $1.60
As other posters have mentioned, driving is a much better option. If you are going with another person or two it is probably the same price or cheaper to rent a car.