I recall estimating the cost of my greenhouse at $10-12k, but that was nearly 15 years ago. The frame was stick-built, with about half recycled lumber. Most expensive part was the polycarbonate sheathing, 6-wall roof (R3.8) and 3-wall sides. Bought foam-insulated forms for the knee-walls and laid foamboard and Reflectix under the floor, which is pea gravel and coarse sand.
It gets really cold here (-34°F a couple nights ago) so I put in a combination of passive and active heating. At temps below freezing, the insulation and passive heating keeps it about 25° above the outdoor temp.
Just replaced the fan-forced electric heater (which had a dodgy thermostat) with a better one that works in tandem with the quartz radiant tubes on the ceiling. I keep the wee propane heater there, in case the power goes out, but haven't used it so far this winter. Nice to have ripe tomatos at Christmas.
Now in its 14th winter, there's only been one loss of plants, owing to cold air leaks in the louver vents, which I now cover with foamboard at night. Here's a link to an earlier thread that might have some ideas.
The thread "Low-tech Solar Radiant Heat" in
Greenhouses forum
