I suspect that if you could provide proof that you had paid for other things around this booking, that you were genuine in your belief that it was a cheap fare as opposed to an error fare, you could initially argue with BA, suggesting that they honour it. If they still didn't, a strongly worded letter with a threat of legal action might sway them. You may want to mention that any settlement is between you and them, and it will not be published (simply to prevent a deluge of other claims). You might be successful.