Who did you book with? Which company? Is it one of the longer crossings? Is it the company themselves who are offering 'Caravans go free' or is it with another company?
I suppose you could say you thought you were getting a good price when you booked, so it would still be a good price if you hadn't seen the free caravans offer! It's hard to know what to do - book early, and then find there's a better offer - or wait, and miss out on what you thought was a cheap deal. Bit like with the budget airlines really - the only good price is the one you actually get!