I think you might find that easyJet's expansion into DSA is nothing to do with the economics of bases. They had an agreement with Peel which limited easyJet's expansion at MAN unless there was also growth at LPL. With opportunities at LPL thin on the ground, easyJet have agreed with Peel that if they expanding at DSA instead, they can still start more routes ex MAN. It's no coincidence that MAN-ZRH, SSH, MAH (an extra based aircraft) have all gone on sale on the same day as the DSA flights.