CDG into Paris without taxi

Depending on where you are staying, Les Cars could work. It's a bus service run by Air France. I found that even taking a city bus to/from the Les Cars stop was better than the metro. The bus is very comfortable and a porter loads and unloads your luggage. Buy tickets from the driver. If you buy a round-trip ticket, you save a couple €. Check their website for route details and timetable.

There are also three RATP bus routes -- routes 350 and 351 that connect CDG to Gare de l' Est and Nation, respectively, and the Roissy bus that goes to Opera. Routes 350 and 351 cost 3 bus tickets and are the cheapest way into central Paris from CDG. You could well be one of very few tourists on the buses on these routes because they are often used by airport workers. Trip to Nation is about 1 hour and 15 minutes from CDG. All three buses, Roissy, 350 and 351 are regular city buses and not as comfortable as the Air France coach mentioned above. By the way, how many are in your group? The cost of the Air France bus if you have more than 3 people could be close to the cost of a taxi.

Further to JHK's information on buses 350 & 351, you can buy the tickets from the driver. Have euros on hand. They pick up at each terminal but you'll need to ask at Airport Information where the stops are, that is, outside of which door to wait. That may have changed since I last use these buses. Schedule is approx 3 times per hour in the daytime.

I would suggest the Roissybus - it's premium fare but non-stop into Paris and cheaper than the Air France buses. The stop at the other end is by the Palais Garnier opera house.

The Roissy for two people would cost over 24 euro, whereas the RATP 350 or 351 cost for two would be about 10 euro. Your total cost depends on the location of your hotel because you will probably want to take a cab from where the buses terminate, in Paris, to your hotel.