Answer from Paris Digest, France
09:34 February 12, 2010
1. Jain, Montparnasse train station is about 30 kilometers from Charles de Gaulle airport. But, in Paris, with traffic jams, we count in hours.
2. The cheapest mode of transport and the fastest at rush hour is metro. Board RER B at Charles de Gaulle airport station and exit at Denfert Rochereau station in Paris. Board line 6 direction Charles de Gaulle Etoile and exit at Montparnasse station. That should cost you around 15 euros.
Enjoy your stay in France.