There are two RATP public transport buses that serve the airport. Buses 350 and 351.Bus 350 is a public-transit bus that links CDG Terminals 1 and 2 with the 'Gare de l'Est' an Bus 351 links Terminals 1 and 2 with 'Nation' in Paris. Although these are the cheapest rates they would not be the preferred choice for travel into the city.
If your flight gets you into Charles de Gaulle late at night the only bus service operating at night the 'Noctilien', which links the airport with various destinations in Paris, line 140 that goes to Gare de l’Est .The service runs between 00.30am and 5.30am. It picks up passengers fromentrance 26 of terminal 1, entrance 2 of terminal 2F and Roissypôle bus stationevery hour. Fares depend on distance travelled, but can range between €1.50 and € 8.00. The trip should take just under 1 hour.
To help plan your trip to your final destination check-out ratp's-interactive-map-herewhere you will find maps for RER trains, buses, night buses and the Metro.