Mile for mile we do have quite expensive taxi's here unfortunately, just had a quick look at the camp site location and it appears to be Kirk Michael so £25 sounds about right.

You could try ringing/emailing a couple of the private hire firms for quotes as a pre booked private hire is likely to be cheaper, but given the time of year and how busy they are its a case of supply and demand i'm afraid, so I wouldnt expect too much of a bargain.