It's totally going to depend on many variables you have not yet specified

Where will you park, at the hotel? How much would that be? At an outlying train station? How much would that be (considerably less than parking at a hotel)? Which route would you be taking? How much would the tolls be? What are you driving? Saying two fillups is meaningless unless we know how much gas you actually plan on using.

The cheapest way is probably the train to New York, then one of the cheap buses to Boston.

Metropark to Penn Station $10 each way. Buses are like $20 or $30 each way depending on dates and time. If cheap is your preferred method, it's going to be hard to beat the bus.

If convenience matters the bus may not be a decent alternative.