The economics of nightstops versus no nightstops at LGW have changed in the last year as GIP have restructured the charges.
Before, you used to pay a landing fee only and that varied based on your time of arrival - so the charges for an aircraft which landed at 2230 and then departed at 0700 were based on an off-peak landing fee only - which was about £400.
As of 1 April 2013, they have split the charge into separate fees for landing and take-off. You now pay a £247 charge for landing at 2230 and a £752 charge for take-off at 0700 as that's a peak departure - so basically for every nightstopping aircraft, your runway charges have shot up from £400 to £1000 (in broad terms).
Assuming easyJet are paying fees based on the published structure of charges at LGW - which is not a foregone conclusion - then with 50 based aircraft x 180 days in the summer season, it's £5.4m extra. Nasty.