What is the cheapest and fastest way to get from Charles de Gaulle Airport to Mercure Paris Montparnasse Hotel?
Take the RER B from CDG to Denfert-Rochereau. Transfer to Metro line 6 (direction Charles de Gaulle Etoile) and take it two stops to Edgar Quinet. Fare: 8.20€.
If you don't want to schlepp your own luggage, take Air France coach #4 from CDG to Gare Montparnasse (one stop at Gare de Lyon). Your bags go in the luggage bay. Fare: 14€.