Hi Pascale,
You do not have to worry about the cost of your selections when using the Disney Dining Plan.
For example, each table-service meal credit entitles you to the following:-
Breakfast
Entree
Non-alcoholic beverage
OR
One full buffet
Lunch/Dinner
Appetizer (Guests ages 3-9 only)
Entree
Dessert
Non-alcoholic beverage
OR
One full buffet
You are free to choose any Entree or Dessert that you like from the menu at any of the restaurants which participate in the Disney Dining Plan. It doesn't matter if you choose the cheapest item or most expensive item, your meal credits will simply be deducted from your Dining Plan according to the number of credits required at each particular restaurant.
When it comes to snacks, all you need to do is look out for the logo shown in the Dining Plan brochure. (Most items under $4 are generally considered as snacks)
You can find out more information on the Disney Dining Plan on the Walt Disney World website
HERE.
Wishing you a magical trip!