I fly this route almost weekly, and i think there is a lot of excess capacity.

The shuttle rarely sells out, and i've often been on the DL/AA flights with only 4 or 6 pax.

AA used to 3 day advance purchase o/w on the route to students/faculty for $68, but stopped about six months back.

There are often good deals ($169 r/t) for advance purchases, but o/w and <7 days advance purchase are extremely expensive.

Curiously, DL sometimes "breaks" the apparent price fixing by offering cheap seats with little advance purchase. US and AA never do that.

I think a lot of price sensitive travellers take UA or B6 from IAD, or southwest or airtran from BWI. Those fares are often $50 o/w with one week advance purchase.

It will be very interesting (and beneficial) to see how B6 entry pushes prices down.