G-BPEC
Its easy to look up the "rack rate" for any airport.
For the lowcost boys its the "bulk rate" for all the airport fees that counts.They are also very interested in the support the airport offers to new routes.The other very important factor if not more important factor is the cost of fuel / handling.Its the total cost that counts.
PLUS do the pax want to use the airport. Easy want to build up at LGW the total cost there has to be a lot more than either LTN or STN but, lots and lots of rich potential travellers live close to LGW.
IF they can get them to pay a bit more and they can keep the turnaround time reasonable then easy can make money at LGW.