the shuttle bus goes right pass the station about every 15 minutes. its only a few pounds

You would be just as well to get the bus. There is a good bus service from the airport (the front of the terminal building), including the City Centre and both Railway Stations (Central and Queen Street) and the bus station. The No. 905 bus departs from bus stop 3 and operates a regular service to the City Centre and Railway and Bus Station.
Route: Airport – Bothwell St/Douglas St – Bothwell St/Wellington St – West George St/Hope St – W George St at Queen St Train Station – N Hanover St – Buchanan St Bus station. On the way back to the airport the route is Buchanan St Bus Station – N Hanover St/George St – South side of George Square outside the tourist information office – St Vincent St/Buchanan St – St Vincent St/Renfield St – Waterloo St – Airport. All bus stops are marked with an aeroplane logo.
Timings: Monday to Saturday 05:40 then every 10 mins until 06:00, then every 7 mins to 20:30, then every 15 mins until 23:00, 23:30, 00:00, 00:30: Sunday 07:00then every 15 mins until 09:30, then every 10 mins until 19:00. every 15 mins until 20:30 and every 30 mins until 00:00, 00:30
It will take approximately 25 minutes to the City Centre. A single fare is £3.50 and £5.30 return.
A taxi would cost around £20 or more.

Black cabs charge a standard rate, which I believe is 20 quid. If you use a private hire cab it’s much cheaper from 9-15 quid depending on how busy the traffic is. You will need to phone a private hire firm and ask them to send a car to the station. Most companies will let you agree a price in advance – 12 quid’s pretty standard. I use Glasgow Mini Cabs – 0141 429 1111.