also, you cerificate provider may charge (if for example you GP does it)
As a rough guide to you, me and my husband recently completed LPA's (PA and PW each so four in total) plus simple wills at a solicitors office.
It cost us 800 quid plus vat - the solicitors office are keeping them until they need to be registered, so this does not include the 600 pounds it would cost to register all four of them. it included the solicitor being the certificate provider. I have no idea if that was an average price or not, because i wanted it done and just went with the first one