From Roissy Airport (Paris-Charles de Gaulle) to Montparnasse train station:
more than 1 hour by RER B train line then metro line 6 (change: Denfert-Rochereau station). The price is around 10€.
From Orly Airport to Montparnasse train station:
40 to 50 minutes by RER C train line then metro line 4 (change: Saint-Michel station). The price is around 5€.
From Orly Airport to La Rochelle-Ré Airport:
There are also connections between these two airports.
The official website to consult is:
By train, from Montparnasse station to La Rochelle station:
The one-way ticket costs around 40 €, for a journey of approximately 3 hours.
Warning: before getting on the train, you must “stamp” your ticket in a machine located at the station!
You don’t need to stamp e-tickets taken online.
By train, from Roissy Airport to La Rochelle train station:
There are also TGV (High Speed Trains) connecting Roissy to La Rochelle with a change of train in Poitiers, find out more on the site: https://www.sncf.com/fr
By bus, from anywhere in Europe
Check out these sites:
from anywhere in France to La Rochelle by carpooling, visit the site: http://www.blablacar.fr