Head to Stubhub's office in the Courtyard by Marriott basement at 114 W 40th St.
The closer to performance time, the lower the various Stubhub ticket prices get. That's how we got last minute seats to DEH on a Saturday night 2 weeks ago (center Mezz D for $225 plus fees.) But seriously-- you have to hang on there till about 15 minutes before curtain time to get the best prices, then race back up to the theater on 45th St to take your seat.