An 'average multiplying cost' might not be much use to you. That's because you pay a much higher rate for the first 728 kWh of electricity (per annum) than you do for any further units. So, for example, someone using 1456 kWh won't pay double what someone using 728 kWh does. (Similarly, with gas, the first 4572 units are dearer than the remaining units).
To see how the system woks, read this:
http://www.npower.com/web/At_home/electricity_ and_gas/types_of_tariff/credit_meter_tariffs/i ndex.htm
Then click the 'Prices and discounts' link and enter your postcode. That will provide you with the relevant costs for your area.
As long as your annual consumption exceeds 728 kWh (which it almost certainly does) and 4572 units, you can calculate the figures for a monthly direct debit based upon about 60 kWh and 380 units per month at the higher prices, and the remainder at the lower prices.
Chris