Next step is take a look at your electric bill and see how much you pay per KW. The national average is $.12.
Lets say you're growing on a 24/0 schedule. 500 watts x 24 hrs= 12,000 watts a day, or 12 KW a day.
12 x .12 = $1.44 a day or $43.20 a month.
So it would cost about $86.40 a month for 1000 watts of light. That 1000 watts of light will get you at the lowest end 250 grams. It takes 7 months to go from seed to flower. 7 x $86.40 = $604.80
604.80/250 = $2.4192 per gram per 1000 watts
or middle ground at 500 grams per 1000 watts
604.80/500 = $1.2096 per gram.
or at the high end at 1000 grams per 1000 watts
604.80/1000 = $0.6048 per gram.
Then you need to factor in the rent/cost of the buildings.