As of this writing (updated January 1, 2013), here are two Chinese bus companies that charge $15 or less for a one-way ticket between these two Northeast cities:
A third bus company, Boston Deluxe also offers $15 one-way rates for transportation between Boston and New York City.
Bear in mind, we're not talking about luxury transportation here. If you're seeking a comfy ride and many amenities en route, you may prefer to take the LimoLiner between Boston and New York.
However, if you're a college student or other traveler with limited means, you'll be among frugal comrades in addition to many Chinese travelers. For an insider's look at what it's like to ride the Chinese Bus, read Eric Gillin's excellent (although dated) account and FAQ, Everybody Fung Wah Tonight!