Here is the reason the rates are so expensive: Red Sox at Orioles that weekend. Red Sox fans have booked the hotel rooms, I would be shocked if there wasn't 20k Sox fans at those games. You can try and wait, but your best option would be either to suck it up and pay the higher priced rate OR stay near the airport. Usually much cheaper rates, free parking and the light rail will take you literally right into downtown (the stop is next to the baseball stadium).
