it all depends how much needs changing - you may be best advised to get the waterpump changed as well (good practice seems to be once every 2 belt changes), & there will be rollers & seals to swap out. A simple belt change could be £250-300, but could rise to £7-800 worst case. I would suggest you do get anything done that you are advised on - For example I bought a 924S several years ago with a nice new receipt for belts. What the seller omitted to tell me was that the oil seals were leaking badly, which I discovered a week later when oil started seeping from the bottom of the covers. Of course the whole job then had to be done again - with new belts.