Caravans aren`t like cars - there is no easy way of determining what is or isn`t a fair price.
Whilst obviously age is important, it isn`t the main factor (as it tends to be with cars). Condition and structure is as important as age, and TBH the best way to determine what does or doesn`t constitute a `fair` price is to do some legwork, visit some dealers and see what is in your price range. That will at least give you an idea of what you should get for your buck, its then a case of can you tow it, do you like the layout and whats the deal?