GLCYZ
It is still a considerable cost that EZY, RYR, EXS etc generally don’t have and is proportionate to the size of the aircraft - plus you need standby cover etc and there is also associated loss of efficiency of crews as they spend their working days (Friday pm, for example) positioning for Saturday earlies.
Taxi costs also add to the total bill.
Stansted was also considerably closer to London City than Southampton is.
Best wishes to SOU and CFE but all the above will hit the bottom line.