I think you are being charged for the new procedure of putting in a post, the original crown would have been charged at the normal rate for that particular procedure. It isn't easy to work out what should or should not be paid for, my dentist always tells me if I can have a repair or replacement free of charge, and it largely depends on the length of time since the treatment. The dentist may well have considered putting a post in initially at a cost of double what you paid, but decided to try and keep the cost to a minimum at that time. Now he seems to have decided that the only way to make sure the tooth remains stable is to put in that post. Dentistry is extremely expensive in this county - I pay �68 each time I go just for inspection, descale and polish.