Given I have a senior bus pass, access to the airport for me is effectively free. However the busses that charge normal punters it's £2 whether it's one stop of 20. So getting to the airport a bus makes sense with no parking charges. Of note there does not seem to be a counter system that shows how many people get off or on the local bus. It no doubt would be different for the National Express or Green Line longer distance coaches that are not capped at £2.