It's likely only legal if they had a policy, of which you were informed or had the reasonable opportunity to be informed (e.g. it was on their website; it was told to you on the phone when you booked the appointment; it was on a sign at their location; etc.) of a charge for cancelled apppointments. Otherwise, an appointment may generally be cancelled without liability; people have to have notice or warning of a cancellation charge, so they can be said to have agreed to it. That said, if they are going to go ahead and sue you for the money, it is likely the better course to pay rather than get involved in litigation because you can never be 100% sure of winning--but you can be guaranteed that if sued, you'll spend at least the better part of a whole day in court, and possibly more.