After last winter I've learned my lesson. I'm heating my greenhouse. Only question is how? I have a $500 budget and I want to heat with 100lb propane tank. This leaves me with $350 for the unit. I was thinking something like:

http://m.globalindustrial.com/m/p/hvac/heaters/vent-free-and-room-heaters/vent-free-radiant-heater-hr25ml-liquid-propane-25000-btu?infoParam.campaignId=T9F&gclid=CLDIqcqUiL4CFU4R7AodVXMAMw&gclsrc=aw.ds

Any thoughts?

I have a harbor freight 10 x 12' greenhouse. I know I need 20-25k btu's. I figure kick on at 40 degrees and off at 50 and let that cycle all winter. I know I'll get cold spots so that should be fine. I thought I'd add a fan for circulation. Any idea how long the propane lasts?

There are heat loss calculators and heating cost calculators on the web. I'll try to post a link.

http://www.littlegreenhouse.com/heat-calc.shtml

For heat cost calculations you need to know the average difference in temperature inside to outside. Cost is proportional to that difference. There is no need to maintain that warm at night when the plants are dormant. Just keep them from freezing. But as spring approaches warmer nights will speed up growth if you want earlier fruit.

I would suggest to leave them dormant in the coldest parts of the year and you can also save on heat this way.  If you would get down to O F then it might make more sense to keep the temp around 34-36 then just have them wake up earlier like half February or so when it could save you on heating costs and you get better light outside.

Make sure you have proper ventilation. A friend of mine had some problems this past winter with leaf burn on the plants he overwintered. He traced it back to the sulfer dioxide byproduct produced by his propane heater. I think a new vent solved the problem for him. 

Is propane the only option?  How about a small wood pellet stove?
I know a local organic farmer (here in NJ) who heats multiple hoop houses all winter using wood stoves.  With a double layer of plastic and the wood stoves, he grows/harvests veggies through the winter.
Jim

What I have seen is that you can inflate the double layer of poly with a drier duct and a wind powered fan, but the draw back is if there is no wind no inflation occurs.  You can also build a smaller hoop house within the hoop house to create a secondary protected climate and it will hold in a lot more heat, the problem being is this is usually done as a low tunnel inside so not too big for trees.  Rocket stoves supposedly do well in the greenhouses and if you add a thermal mass to it you can maximize your heat retention also saving on fuel.  I imagine if you have an exhaust on any heater you could run the stove pipe through it and save the thermal mass though.

With electric heaters and this COLD winter, my 8x6 greeenhouse added about $100.00 / month to my bill during the worst parts.
I kept thermastat set at about 70F

Propane is easy. Auto start and no maintenance except for maybe filling the tank 1x in the winter. So what kind of unit to use?

http://answers.northerntool.com/answers/0394/product/27977/procom-vent-free-trifuel-blue-flame-wall-heater-25-000-btu-model-mtf300tba-b-questions-answers/questions.htm

Looks like you start it manually and from there it keeps a piliot light lit. Once it reaches the temp desired it "kicks on". Sweet!

I'm looking at this  http://www.bensoninstitute.org/Publication/Manuals/Walipini.pdf  as being the best solution for me. Of course I have a tractor with backhoe and front end loader to dig it with, and a nice south facing slope where I can dig it in but still be high enough to put a french drain under it. But, that would be more for a winter garden. For figs I'm thinking more a trench up on the top of the hill that would keep the roots and trunks down where the geothermal effect would keep them at a constant temperature, and then do a shallow hoop greenhouse over the crowns. Let them get enough cold to go dormant then put a layer of straw and a tarp over it all for a thermal blanket to get them through the bitter cold. You could wake them up early in just the greenhouse but have them protected from the last few frosts. Then take the cover off for access in the summer. You could have big trees at a convenient height for picking, and protected enough for even the harshest winter. You can still do all the bells and whistles with drains and irrigation but save a lot on structure and heating.

Going down instead of up just makes a lot of sense to me.

Kerosene Tube heater.... Worked well for me this winter and the plants you saw when you stopped by. Just have to keep the tank full. I think that it's better to heat the enclosure to 60 and let it cool down to 20 or 30 and then turn it on again. The start up on these kerosene heaters is dirty but they are fairly clean when they are running.
My plants wintered well. I have figs on all three now and they were frozen solid for months.