I'm also flying into boston that same week. if you want cheap lodging the best place to look would be off the main subway lines (the south end is nice (where the campus is), not sure about prices...don't look in the back bay, very pricey)
i would recommend taking the subway close to campus and walking the rest of the way (bc theres no actual stop right there, its really cheap..3.50 a ride i think)
when you scheduled the BU interview, did you actually talk to someone, bc i emailed and never received verification...so im a little nervous