There is a fig operation in Rhode Island growing figs in a greenhouse, I believe they wrap their fig trees with some kind of perennial ground cover during winter. Wishing Stone Farm is the name.

I would be interested to know about the quality of figs ripening in a greenhouse. I have a small Riga greenhouse and have found that the few figs that have ripened for me this fall have definitely ripened differently from outdoor figs, longer ripening time and less soft. More on the drier side, with possible spoilage before ripening completes. The fruit has not come anywhere near "dead-ripe" or ripening to perfection.

Thanks Rafael!  Wishing Stone Farm looks like a decent operation!

Steve, 
As you can tell I know very little about greenhouses and coldframes.  I didn't know the distinction between coldframes and greenhouse.  Thank you!
I am looking at a 20' length tbd coldframe with double poly and roll up sides.  There is a hot water natural gas heat system they sell that pumps warm water throughout and could keep the temps just above freezing but I don't think I can justify the cost.  
I would love to find a passive heating solution just enough heat so I don't have to cover the figs in winter.  The goal is to extend the season and get figs growing by May and ripening until Nov 1.

Pino:

There is no cheap way to heat a greenhouse in winter. Passive won't keep it warm enough at night and any covered structure over heats on sunny days. Your best bet is to cover for winter protection as you are accustomed to doing. Then 2-6 weeks before last frost cover the coldframe to warm the days. I say 2-6 weeks because it won't cost much to heat at night in April. If you can heat on the cold nights in spring you can start a month earlier than without any heat. With some heat at night in spring you might get a months jump on first harvest. Do the same in fall to avoid freezing until about your Nov 1 date.

By heating on cold nights spring and fall and covering to warm days you should gain 4-6 weeks longer harvest season.

Steve

Thanks for all this because I am ready to order a coldframe and it will save a lot in the long run if I can get it right to begin with. 

I like your suggestion to "cover for winter protection as you are accustomed to doing. Then 2-6 weeks before last frost cover the coldframe to warm the days".  I think this method will work fine in my area for cold hardy type figs.  I did a version of this with my dalmatie fig and it worked great this year even with the brutal winter.

With a permanent GH I am hoping to avoid all the work associated with covering/burying the figs.  If in winter I could heat just enough to keep from freezing and managing the sunny days with ventilation (roll up sides). 
Then my ongoing cost should be the heat required to maintain 32-35F.   Is there a way to estimate how much this will cost.  Maybe by estimating BTUs consumed?

Greenhouse Megastore has a heat cost calculator. But that's for normal greenhouse coverings. So you could calculate spring and fall cost there. That should be very little all depending on how much you want to jumpstart things. In winter you need a much higher R value insulation. Maybe an inner layer of some real insulation, inside the greenhouse, over the figs with enough room to put a heater inside, under insulation. In effect a highly insulated greenhouse inside the full greenhouse. There are many possible insulation materials. I saw one on here not too long ago using a very heavy frost blanket.

Have you considered a pit greenhouse? Does your soil drain well? Could you dig a 3-4ft deep pit under the coldframe? That would add much cold protection. Basically just cover that with heavy insulation in winter. Put some of those gutter heating cables underneath for heat. The cables have a thermostat. They are used to clear ice out of house gutters so are weather proof.

That megastore seems to have everything you need for a greenhouse.  Will check them out.

A pit greenhouse another interesting idea to follow up on.  Thanks Steve