'Fees' added on are stealth fares. They go to the airlines and bear no resemblance to what the airport gets.
As far as what airports get for pax charges - well go and look at the published rates for the regulated airports. You can bet that low cost carriers are paying less than that to regional airports. So less than £5 certainly.
As for costs of concrete - that's down to building contractors not the airport, surely? Remember that airports are heavily regulated by the CAA in terms of safety requirements for airside infrastructure.
But I wasn't trying to stick up for airports particularly. I'm just making the point that airlines reducing what they pay airports will not necessarily flow through to the passenger - they'll just have to pay more in other areas to make up for it.