There are 2 RER B stations in the airport, Aéroport Charles-de-Gaulle 1 which serves terminals 1 and 3, and Aéroport Charles-de-Gaulle 2 located next to terminal 2.
One way ticket to Paris costs 9.50€. You can buy it at the entrance of the RER stations or in any metro ticket office.
It takes approximately 30 minutes to reach Paris from the airport of Roissy-Charles de Gaulle.
4 stations of RER B are located in Paris, Gare du Nord, Châtelet – les Halles, St-Michel – Notre Dame and Denfert Rochereau.
AIR FRANCE BUS
There are 2 bus lines, now called Le Bus Direct, which makes the connection between the airport of Roissy – Charles de Gaulle and the centre of Paris. It is available to everybody, even if you are not flying with Air France.
One way ticket to Paris costs 17€. You can buy it at any Air France office in the airport, or directly from the bus driver. More details about Le Bus Direct shuttles
It takes about 1 hour to reach Paris from the airport of Roissy-Charles de Gaulle if the traffic is not bad. Departure every 20 minutes from Terminal 1 and 2, from 5.30 AM until 11.00 PM.