It depends on which day , at which time and
the traffic.
I would look fo Taxi Parsien.
Cost could run E50 + depending on the
number of people and how much luggage. Don't take any taxi that is offered to you by a person standing on the forecourt - go to the stand and take one parked there
The RER/Metro would be cheaper. Take the RER-B direct from CDG to Gare du Nord.
the last time i went to cdg airport and had to go to gare du nord, i a sure there was a metro that we picked up at cdg and that droped us off at gare du nord without having to change trains. this was several years ago and things may have changed.