We arrived at Charles De Gaule airport Terminal 2F, and it was a 10 minute walk to the airport train station (you walk indoors, and can use a luggage trolly the entire way).
We went to the ticket counter and asked for the next TGV to
Disneyland Paris Resort, which is also called the "Marne-la-Vallee / Chessy" station. I'll have to check my notes or receipts to see what the price was. There were two prices, one for first and the other for second class. We opted for second class, since it was only a 10 minute ride on a high-speed train.
We generally find that train travel in Europe is very reasonable. Last month we visited Rome and Paris. The cost for the train from the Rome airport (FCO) to the city was 9.50 EUR. The cost of the RER train from CDG to Paris was 8.00 EUR.
Woody