Local train from Disney Paris to Paris

Take the RER A train, cost is about $9.00.

EDIT: per Sam's post it's €7,60 - which right now is about $9.50.

It is 7.60 EUR and it is the only train that will get you from Disneyland to central Paris and they go about every 6 minutes in the late afternoon.

There are no round trip tickets on the RER. You buy two one ways or a day pass or if you will make the trip often, a Navigo Decouverte. Always buy two tickets if you know you will be making a 'round trip' so you don't have to fumble around buying tickets late on your return. A Mobilis day pass is a good deal if you are going to Paris for the day as you can then move around Paris on the metro on the pass as well as to and from Disney. A 5 zone pass is a bit more expensive than two one way tickets but you probably break even with further travel. Note that you have to clear the gate on return before midnight -- your pass turns into a pumpkin at midnight and you can face a big fine for not having a valid ticket.

I have edited my post per Janet's post. I saw a website offering round trip tickets but I misread it. My apologies.

Karen it is a very easy journey to make as a day trip and considering the distance the train is very inexpensive.
Amazingly the train station there is literally at the gates to the park ; there is nothing at that stop except for Disney so it is like getting dropped off at the front door.

We walked to the train station Châtelet - Les Halles to catch the train to Disney in the morning, stayed all day and for the firework/light show which was excellent and we were back in Paris that same night.