I remember you posted before about this - wanting to go in May, right?
Most of the May NYC-Boston Megabus dates just got posted on their website. If you want to see which runs still have the $1 fare available, just choose "one passenger" when it asks how many people are traveling. Just change it (swiftly) once you've figured out which of the trips you'll be booking.
Otherwise, it will (in most cases) be $1 for the first traveler and $9 each way for each additional one. (That is, booking now for May. The prices will go up as the travel dates get closer.) I grabbed my usual tix for my spring Megabus trips and got my $1 fares! The first seat booked on each run is the $1 seat.