Weedwhacker said:PML, not to be a wet blanket -- but unless you expect to generate considerable income from the plants in the GH I think you are likely to find the cost of heating one prohibitive. 
Could you set up some shelves with fluorescent lights to grow the plants?
Spot on. It is expensive to heat my greenhouse in March let alone through the winter. I have a little one I keep warm all winter long that is 12x16 attached to my house. It is primarily for starting seeds and I have plants/seeds in there starting in November and I am in zone 5. I couldn't afford to have a greenhouse if I didn't sell plants from it. the supplemental gas bill alone in March was 190.00, I also have a wood burning furnace that we fire up every night. most of that is night time heating (really, early morning, as I don't go keep the fire going) , very seldom during the day does the furnace run.
Some greenhouses rent space for the winter since they have to keep it warm already.
The other thought is led lights for your plants inside. They are coming out with some new plant fixtures that work well. My neighbor has an experimental led light/fixture her daughter in law is trying to start a home based business with so she is trialling it for her, set up in her basement and her plants were beautiful. even her african violets which never bloom for her set bud for her under those lights.
