If you're paying around 48 to 55 euros per adult, then thats the full price of the meal, so you won't have anything else to pay when you get there. Sorry I don't have this years brochure, so I don't know what the advertised brochure price is for the Princess dinner.
As far as I know, you either pay the full price in advance, or nothing. You can pre-book meals at reservable restaurants without actually paying for the meal - you're just making a reservation, and this costs nothing (well, it does with the Disney reservation number anyway). But I don't know if this is possible for the Princess meal.
Elaine