Valid point ink, but in theory Translink (+DRDNI) and the Airport. Two years ago an intercity rail station and bus terminal opened in Newry... which was manned and had a 300 space car park. It cost a grand total of £14.7million and has two platforms long enough for 9 carsets. What I'd propose is a non-manned low-cost no frills station to be built on land already owned by NIR. Perhaps platform space for three coachs (if staying budget) or possibly four (if you plan ahead.)
Not being crazy, I say you could do it for under £6million. Now say it does achieve 1million pax per annum which is possible if you look at Prestwick. And they spend on average £2.50 one way, that route could generate £2.5million per year, without adding extra services. It could be serviced by Bangor - Belfast/Portadown trains, which means no expense made on extra train services to and from the airport. This could techniquely pay it's way in under 3 years, whilst increasing transit from BHD, improving rail ridership figures and eleviating traffic from the Sydenham Bypass which is possible going to require widening due to traffic. (I would set the station far as possible of the road.)
Yes my calculations are crude, but there is logic to this, and I'm not just crazed person shouting about the need for a train station without any thought for how it'd be financed or able.
One last note, the DRDNI (Department for Region Development whom are responsible for NI's infrastructure) have just recently been provided with an extra £108million. And Conor Murphys not really commited this to anything yet. So even he could pay it, unless it all goes to a bypass in the North West.