Best way and cheapest way to get from Charles De Galle to Gare de Lyon train station
RER B train from CDG Airport to Gare du Nord, then transfer to RER D direction Corbeil-Essones or Malesherbes or Melun until Gare de Lyon, is likely the simplest route. Chatelet les Halles is another transfer point you can take, but it can be daunting due to the station's complexity and size. If going to Chatelet les Halles, take either Metro Line 1 direction Vincennes, Metro line 14 direction Bibliotheque François Mitterrand, or RER A direction Marne-la-Vallée Chessy or Boissy-Saint-Léger until Gare de Lyon.
You may use the same ticket for the transfer onto the connecting Metro or RER train.