I would consider this a fair price for replacing the condenser and regassing the entire air-con system. The condenser is a very expensive part and you can pay up to £1000 for a replacement on certain models of car. The condenser is liable to fail if the air con is switched off for long periods or it develops a leak. More often than not, the condenser will seize up due to being starved of oil. That's why it's important to use the aircon regularly to ensure it self lubricates and avoids problems such as this.