This is what I did last year: If you go to
www.bostondowntown.doubletree.com and make your reservation using Corporate account # 02657456, you get a king non-smoking room for $99. Just print out the interview invite email to show them when you check in to validate the rate. It's technically for Residency interviews, but the hotel doesn't know the difference.
It's still not dirt cheap, but it's literally right next to the Med Center, a 2-3 minute stroll down the road and you're at the door of the school. I'm not sure if it still works, but I'll be trying later today and let you know.