The RER B Paris city train travels from CDG Airport (Paris–Charles de Gaulle) to the Eurostar train station in Paris, Gare du Nord, in 30-35 minutes. Please note that there is no Eurostar train station at Charles de Gaulle Airport. All Eurostar Paris to London trains depart from Paris–Gare du Nord station, also known as simply “Paris Nord“.
There are two RER train stations at CDG Airport, one serving Terminal 1 and 3 (see the Terminal 1 to Paris photo guide for detailed instructions), another for Terminal 2 (see the Terminal 2 to Paris photo guide). Both stations will have RER trains going to the Eurostar station in Paris: Gare du Nord.
Tickets for the RER train from CDG to the Eurostar station are 10€ (as of January 2015), purchasable at both CDG train stations, either from ticket vending machines or ticket windows. These tickets are simply “To Paris”; there is no special ticket nor price for CDG to Eurostar station at Gare du Nord, which is within central Paris in the 10th arrondissement.
The first train leaves CDG Terminal 2 station at 4:56, arriving at Gare du Nord/Eurostar station at 5:20. Frequency of trains varies from 7 to 15 minutes between trains for the better part of the day.