I think Ernie has said that the cost somewhat depends on your
scrounging ability... Some can't find barrels in their locales.. I can get them free if i look around or can get them all day long for a few bucks.
Wood heat is dry (as long as the exhaust goes outside). The wood heater will draw cool dry air inĀ and gaseousĀ moisture out through the exhaust.
So, if they were heating with gas and exhausting the fumes inside the green house, they probably did have big condensation problems. The rocket mass heater (exhausted to the outside) would make moisture less than if they used electric heat..
You can look around the internet for years and i doubt if you will find anything that will heat more efficiently than the rocket mass heater...