Assuming this is a Y booking, I'm not surprised at all as the actual BA fare could even be $10 or so. The $400 YQ is the problem here (and the fact that you still have to pay it when booking using miles).

Actually what is even worse is that AS charges $540 YQ which is higher than the revenue booking. That is unusual as it should be $400. But even if it was $400, Y long-haul reward bookings on BA are a particularly bad use of AS miles. Probably the only exception would be if you need a fully refundable ticket as (for revenue) those will cost easily more than double the nonrefundable fare.