CAA scheme of charges for ARC renewals/validations is here
http://www.caa.co.uk/docs/33/229Airworthiness.pdf check out pages 16 and 17 for what the CAA will charge for ARC renewal. The price quoted is just for the renewal, any further investigations will be charged at £187/hr.
The CAA don't have enough staff to be renewing ARC's, hence the reason why it is being put out to qualified individuals at approved organisations.