You do not have to buy a pass before, if you like you can just pay once you board the bus, Im not sure of the current bus fare rate but if you click on the link that I have provided you can find out how much its going to cost. Again you can pay once you board the bus, just be sure to have the exact amount. Click on the link and you can see what buses/trains you can take. you can select the attraction/landmark and just chose Union station (Start) and Museum of Contemporary Art (End) Hope all works out!
I m not sure if they sell passes at union station, but you pay on the bus, it s $2.25 and there s a machine right when you enter where you put in the money.
$2.25, as soon as you get on the bus. Have exact fare, no change given.