Hi Tubbs
Without having anything in writing. it would be difficult to prove either way. You can't try and stop the payment with Visa. I also think going to the small claims court wouldn't be a runner, because you can't prove the price quoted either way.
You could find out who is the overall manager of the hotel, and write a letter to them expressing your concerns,what you were originally quoted, and how you were taken aback by the extra €200 charge, that you had not expected. You can say that you understand that someone can make a mistake (the person who originally quoted you), but explain that you wouldn't have held the event in their hotel, if you had known that the price was going to be nearly 50% extra per head. You could also explain that you were unable to address this issue at the time of payment, due to family being there, and you not wanting to spoil the day.
It might be better to go down the route of appealing to their sense of good customer relations, i.e. down the 'good-will' route, rather than going in all guns blazing (which probably will not be successful).
I'm sure they would rather not have very dissatisfied customers, who will no longer be able to send business their way .
Often times business will prefer to keep your future custom, especially if this is a hotel that does lots of 'events' (and I'm sure you and your family will have further christenings/communions/birthday parties in the future, if you know what I mean...). And you would really hate not being able to put that business their way, which unfortunately would be the case if they are not able to come to a mutually satisfactory resolution of this issue......
Nicola