Check your power bill for your price per KWh; this is the price that you will pay for using 1,000 watts every one hour. Multiply that price (in dollars) by 24 (for hours) and then by .4 (for only 400/1000 watts) and you will have your answer.

I believe my last bill was circa $0.10 a KWh, so that would be .1 x 24 x .4, which would equal $0.96 a day.