It is possible to use the local bus to Pleurtuit to get the airport if you do not mind the short walk from Pleurtuit to the airport.
From the airport walk down the approach road to the crossroads, less than 10 minutes. Here turn right and right again and take the footpath which is a converted railway line. It is straight and flat all the way, and mostly tree lined to give some protection from the sun. Follow this (there is a bridge over the main road) until you come to a road. Turn left here and you will shortly be in the centre of Pleurtuit. Left at the round about to the bus stop. In all about 30 minutes from the crossroads.
If going the other way - from the round about take the road past the church and you will shortly come to the footpath on your right.
Bus No. 7 (illenoo) times from Pleurtuit to Dinard are 10:30, 11:30, 12:35, 15:37*, 17:36*, 18:37, 19:37*, 20:27*.
* These do not run on public holidays.
When you get to Dinard cross the road and take the 16a to St Malo This runs every half hour.
Both bus fares will cost 2Euro10 which is a lot less than the probably 30+ Euro from the taxi.
If you have to wait the cafe on the corner by the roundabout is good value. Last time I left I had their express formula lunch before walking to the airport.
The No.7 runs the other way to Rennes via Dinan, leaving at 7:09*, 9:13*; 10:14*; 12:13*, 14:13*, 17:13, 18:13, 19:13.
The 16a goes via the St Malo railway station to the old town. There is a taxi rank by the station.