I've just had a look and compared dates for a LHR-AMS-LGW trip I'm doing in a few weeks and its £22 cheaper than what I paid! If I'd booked the flight today on BA, it would be £25 than what I paid, so this is a saving of £47.
Part of the the difference seems to be in the tax rates. BA quotes £31.90 and AA quotes £18.50. I know that there are some tax loop holes about with regards to ticketing etc, but thought it would of been more on par.