You could take the bus here
http://www.greyhound.ca/home/
It is great you could find lots of covoiture here . Amzing KJIJI !! aHAHAHA....
http://montreal.kijiji.ca/f-boston-communaute-covoiturage-W0QQCatIdZ5QQKeywordZboston
And if you found someone do the covoiture . you could find the price like this http://montreal.kijiji.ca/c-communaute-covoiturage-Montreal-Boston-Friday-9th-April-at-about-1pm-W0QQAdIdZ197261152
Just 30 euros..Great !not so expensive!