The last RER train from Aeroport Charles-de-Gaulle leaves at 23h48. Then there're night buses (Noctilien) leaving twice an hour to Gare du Nord. I was able to find that on the website http://www.ratp.fr .
However, I didn't find which ticket (and at which price) I need for that. So that's basically my question. As well, if there are other reasonable options for getting downtown, they would be interesting for me as well of course.