Ryanair fly Shannon-Beauvais a few times a week (?) and the door-to-door travel there and back cost me about €120 if I include my petrol to and long-term parking in Shannon. Long term parking costs €9 for 24 hours or any portion thereof. Here's a nice link to the
Shannon Airport site.
The Ryanair shuttle bus from Beauvais airport costs €13 one-way and the airport is located 80km north of Paris in fine rape-planted Parisian culchie hinterland. You land at Port Maillot to the West of the Arc de Triumph and the metro in Paris then costs €1.50 for a trip anywhere in the city. I don't know any details about the train from Beauvais to Paris - if you were bent on sparing the pennies you'd have to walk into Beauvais to get a train/bus first. I'll do that next time.
The first thing I noticed was the great spaciousness and the proliferation of Twingos.. !