Take the RER A line and be in the city in about 30 minutes. A single one way ticket, a billet Ile-de-France, from central Paris to Disney costs 7,50 (as of January, 2014). You can buy these tickets from any metro or RER station ticket window or from the automated ticket vending machines (using coins or smart-chip enabled credit cards) within stations. You can start your journey from any Metro / RER station within central Paris (Zone 1) using this one ticket and simply transfer to the RER A train line if youre not already on it. You dont need to buy a separate ticket just for the metro portion of your trip. The normal metro ticket t+ will not work for this trip since Marne la Vallée / Parc Disney is in Zone 5.