ccwood,

First, I strongly urge you to read:

http://extension.unh.edu/Agric/AGGHFL/OFAlight.pdf and
http://www.specmeters.com/pdf/articles/A052.pdf.

This will give you an idea of how much light you really need, though you will need to get a light meter to see how much light the plants are getting inside the GH.

If you want to determine how much it will cost, look at your electric bill. It will have the cost per kW hr. That will tell you exactly how much it will cost, per hour, per light. You can figure somewhere between 12-17 cents per hour, per light.

You don't list where you are located, so heating costs may or may not be a factor.

Mike

A follow-up to my response:

I wrote to Paul Fisher, who helped write the article and he told me about a book he helped edit: "Lighting Up Profits." It's for greenhouse growers and though it will probably deal more with flowers than veggies, the basics are the same. The cost (with tax and shipping) was $35, a steal if you can get extra produce or bedding plants from your GH.

Mike