Quite a few variables in there, even from one operator to the next. The trip cost (ACMI) on an average B737 could be in the region of GBP 4000 plus fuel for GBP 2-2,5K. Then there´s handling and landing charges, plus all the sales and distribution cost plus overheads. Probably GBP 8000 would break even for some operators and constitue a significant loss for others.