Hi, Jennifer! Dinner at Cinderella's Happily Ever After Dinner at 1900 Park Fare sounds like a great way to spend an evening!
The price for this all you can eat buffet is $35.99 for adults and $17.99 for children. Prices do not include tax or gratuity, and a "holiday surcharge" may apply (usually about $2 - $5 per person, charged during busy times of the year at buffets and character meals.) And, though I know you want an exact price, I have to say the prices are always subject to change.
Since you are on a budget, have you considered the Disney Dining Plan? If you plan to have at least one table service meal per day, you may find it to be cheaper to do the dining plan (especially for kids ages 3 - 9. Just one meal can cost more than the dining plan for that day!)
You might want to call a Disney Reservationist at (407)W-DISNEY to get a quote to add on the Dining Plan to see if might save you some money. Click
HERE to read more about the Disney Dining Plan. Click
HERE to see some special offers that might also save you some money!
I hope this information helps! Good luck planning your magical vacation!