Here are all the means of transportation between CDG and Paris:
http://www.adp.fr/webadp/a_cont01_an...cle=X500520521
Some comments:
- the RER B runs through the rough northern suburbs of Paris. As a streetwise French-speaking male I still do not feel especially comfortable on those trains late at night. The express trains do not stop between the last city station and the airport, and are therefore less likely to carry troublemakers.
- the RER stops running around midnight
- taxis run €30-45 to downtown; a viable option for a party of 2-3 late at night
- RER and Roissybus run about €7.50
- Air France bus runs about €10
- if you are around the Etoile area, the Air France bus is faster than the RER, and definitely safe