Cost of Electricity on CL's

To try and prevent tripping a EHU on any camp site we invested in a Owl Electric monitor  http://www.theowl.com/

Now this device allows you to also program in the cost per Kw that will then indicate the cost of the electric you have used either that day, week, month or even year.

Now we have toured both abroad and in the UK for the past 5 years so have no electric company account history to obtain this information.

I then asked a close relative what he pays however his statement shows a charge for the first 'so many' thousand kw's then a lower price for the remainder.

So how does a CL owner charge for electricity used or does anyone know of a link to a site that can supply this information?