Only ever travelled via Dover/Calais ferry, which is always peanuts for us, as we normally book it though the holiday company (Eurocamp normally). I think it only adds about £50 to the cost of the holiday (including the return crossing). It's very flexible as they have crossings every hour, so if you get there early then you can normally travel early.
We're are only 2 hours from Dover, though, so it's easy for us, and only a 1.5 hr crossing. But I'd rather drive than be on a ferry. We did it last year when we drove down to Austria, through France and Germany...
It was a big drive though. Just me and the wife. Not sure I'd like to do it with kids, so you may think it's worth paying the extra to go further on the ferry, but the cost seems to sky rocket beyond a certain point.
Edit...just noticed you said you have a caravan, which we don't. So I guessed you'd be paying a lot more than we would.