The Glasgow Airport Rail Link shuttle between Paisley St James and the terminal stopped in 2014 and the trackbed now carries freight only; anything promising a train from GLA is a decade out of date. The station you CAN use is Paisley Gilmour Street, reached by bus 757 — or just take the £2 Service 500 into the city.
First's £2 single cap (including 'Adult Airport') was announced effective 7 September–30 November 2026; the pre-cap fare on the airport's own timetable leaflet was £8 single / £30 multi-day. After the window the airport service can revert — note gov.uk's national £3 bus cap excludes airport bus services, so there's no guarantee the £3 backstop even applies to the 500.
The airport's live advice page: major works on North Hanover Street pull the 500 off its central stops — towards the airport, North Hanover St and St Vincent Place aren't served (use Bath Street opposite Sainsbury's / St Vincent Street opposite Lloyds); towards Buchanan Bus Station, Queen Street Station and North Hanover are dropped (alight on West George Street for Queen St). Arriving expecting your usual stop means an unexpected walk.
Only white cabs rank outside the terminal; every pre-book/app operator is pushed to the paid pick-up/drop-off zone beside Car Park 2 (£7/15-min zone charge for the driver — reflected in quoted fares), and the airport blog warns drivers 'can advise which zone they will collect you from'. Circling kerbside with the app pinging gets you nothing.