CDG to my hotel, 54 Rue Monge Paris

Train is most economical but if you're not comfortable with that, then an official taxi.

The absolute most economical way to go would be to take Bus 351 to Nations metro station and change there for bus 86 towards Champs Elysees and exiting the bus at the Cardinal Lemoine stop if you are traveling between now and March 5. I think the closer stop is closed until March 5. That trip will take about 1 hour and 45 minutes and should cost 2 euros per person. Bus 351 is not really for tourists. It's more for people going to work at the airport but there is no rule that you cannot take it. The RER trip mentioned above takes about 1 hour and costs 13 euros per person. A taxi will cost 65 euros.

Cheapest will be the combo of RER and Metro. Probably the RER B connecting to the M10 at St Michel/Notre Dame

https://www.bonjour-ratp.fr/

Fixed price taxi from CDG official taxi queue would be about €65

The cheapest is absolutely what JHK said -- funny as I thought of it myself as I live along the 351 (and the 86) and have taken it a few times, mostly to the airport. But i really wouldn't recommend it for a casual visitor to Paris.

In all seriousness though, the more serious more economical option, the RER in from the city, involves either a 15-minute walk from the Luxembourg RER B stop to your hotel, or a change to the 7 or 10 metro line to get you closer to your hotel. That however almost always means stairs in the metro changing and coming up.

You could also do the Roissybus to the Opera area and then take the line 7 metro from there.

A great way of looking at your transit options is to download the CityMapper app and tell it you are in Paris and put in your beginning and end points. It will show you several different options, tell you how long each should take under normal conditions, and even tell you the cost !

Thank you all! What do you think of the Roissybus? Is this different than the one suggested above?

The Roissy Bus is not mentioned above. It makes little sense to take it to get to Rue Monge. It goes from CDG to near the Palais Garnier on the Right Bank. After you get off the bus, you would need to get a bus, the metro (Line 7) or a taxi. I guess you could walk but that would take about an hour.

By the way, when I stayed at Hotel Monge which is across the street on Rue Monge, I preferred to walk to St-Michele-Notre-Dame to catch the RER because I hate being underground so I try to reduce that as much as possible when taking the train. So you have options on where you would exit the RER B if you take the train.

Will you just have flown overnight? This is one time I prefer a taxi and make up the expenditure difference somewhere else?