That makes sense because BA would charter it out at cost plus a percentage and the charterer carries the risk of not filling it up.
For a scheduled flight, BA have to assume a certain number of empty seats and price the remainder accordingly to cover the costs plus the margin.
A one off Bay of Biscay charter in 1996 was roughly £100,000 but the main charter operators had bulk buy deals and were selling the seats for something like £900 at the time, which suggests they were paying about £80,000 for the charter which in turn implies the operating costs were in the region of £70,000. Not sure what an Atlantic charter would have been, but I think you get the idea?