From Washington DC to Boston MA
US Bus Station.com, Your Local Bus and Coach Route Database

The best way to go to Boston MA from Washington DC is to take a bus from DC to New York City and a second one from NYC to Boston.
The average cost of ticket from Washington DC to NYC is $20. ( one way)
Departs are every hours from 7am to 1:30 am
Average Duration: 4 hours.

The average cost of ticket from NYC to Boston MA is from $15 to $30 (one way)
Departs, hourly, from 12.15am to 10.30pm,
Average Duration: from 4 to 6 hours.

So for wahington DC to Boston Massachusetts in bus :
Average cost: from $35 to $50 one way.
Average duration : 8 to 10 hours

Only Greyhound provide a solution for $49 (web only fare) duration from 9h50 to 11h50 but you still have to 2 buses and stop at NYC