Becca,
The answer is as variable as the plants.
Size, shape, covering, how well sealed, minimum inside temperature, outside temperature, wind, angle of the sun, type of heater, & cost of energy for that heater all effect the heating cost.
In Boise, I'd bet it would be pretty pricey. I'm in southeastern PA, and the 14' x 18' greenhouse my collection is in costs me about $1600 per winter to heat.
(This is something I'd like to do - ) I read an article in the AOS BUlletin once about a guy who put a couple of hundred feet of large-diameter, unglazed, clay pipe about 6 feet underground. There was a filter/screen at the entrance (disguised in a small "well head building"), and the outlet contained a fan pulling the air through the pipe into the greenhouse.
As the ground is naturally in the low- to mid-60's at that depth, the incoming air kept the plants comfortable in the winter, and served as cooling in the summer - all for the ongoing cost of running the fan.
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