I read a cbc.ca article that said the airport bus is the normal $2.75 from downtown. If you catch the bus at the airport and go downtown it is $8.
It also said that if you have a day pass or a monthly pass you do not have to pay extra. So if you have a monthly pass, I don't think you have to pay anything. (other then your monthly pass)
http://www.cbc.ca/news/canada/calgar...us-direct.html