Parking at one of the train stations is free if you buy or already have a ticket but you need to be there before 8am or you'll never get a spot. You're looking at 3 zones from Coatbridge Sunnyside (£99.40 a month) or 4 zones from Airdrie (£118.40) and both options will be easier and probably cheaper than trying to take the car into Cowcaddens.