Surprised no company has complained about this yet. If I negotiate a 10% discount I expect 10%, not a random number between 1-10% based on how BA decided to divide up base fare and surcharges.
I was about to mention this! Awhile ago I saw a discount of something like 50% on an Africa-US fare. Well, it turned out that the base fare was like $150, so the discount was $75. It dropped the total price from something like $1,000 to $925. I mean, I'm not going to complain about a $75 discount, but it's not the windfall that the 50% off number would have you believe.
Note: I don't remember the exact numbers, but it was something along these lines.