Hideously expensive - cheaper to buy a new car than park there.........the Council are really cashing in on the parking and they can't stop re-arranging the roads either.
Go to the short stay car park, (�7 for 3 hrs I think at present) not the 'rapid' pick up one - that stuffs you �10 for 3 hrs.
They have pay as you leave machines installed now, so do try to stay under the time if you can....machines take notes as well as coins. The 'rapid' car park is free for a 10 mins pick up only so if you can, just park up and then when you need, go to the terminal for the arrival time only.
There is a car park you can stop in for free at the community hall 1 min from the airport on the corner of West Lane - its on the left before the roundabout / entrance to the airport if you are coming from Bristol direction, on the road to Felton. Don't know if thats any use, but I stop there if I need to wait at all for anytime or if not using a taxi to get to the airport.