I really didn't find eating all that expensive. Yes, more expensive than buying snacks from a supermarket or shop but similar to eating at any other event or day out venue.
There's a shop in the station where we bought crisps and wine (for the train back to Paris). We ate Pizza slices inside the park and had ice-creams etc too but I really don't think it was overly expensive. I think the biggest issue is how busy everything is, so sounds wise to book.
I should point out that we were on a hen do for my last visit so you may want to discard my advice totally!
Take your own snacks with you. MacDonalds is cheapest place to eat, massive queues at times though.
Wise to book in advance for restaurants they get very busy and you can be waiting an hour just for a table.
If you are going outside school hols will be less busy. I have heard there is a snack shop in the train station.
I haven't been to the shopping centre as prefer to spend our limited time at Disney there, but have heard that the supermarket there is good. Depends how much time you want to spend travelling to a shop.
For lunch, these are crepe vans etc in the park for a less expensive snack.
We struggled but then a friendly local told us about this, I feel Disney staff wont tell you so you spend more on site but I am possibly totally wrong. http://www.tripadvisor.co.uk/Attraction_Review-g226865-d668695-Reviews-Val_d_Europe_Shopping_Center-Marne_la_Vallee_Seine_et_Marne_Ile_de_France.html
Please check its the correct link as it was a few years ago, it was a massive shopping center a few stops away on the train, not many people spoke english but we managed with our english pointing and basic french. Oh they sell beer too.
We are planning on taking DS (nearly 4) to disneyland paris in may. I've heard eating out can be quite expensive so where are the best places to eat? We'll be staying bed and breakfast so it will just be dinner/tea/snacks we need to worry about.