I thought with propane the thing was not so much the gas going bad as it was condensation and such in the generator / other devices so you had to exercise them every so often. Use a bit of gas but worth it.
So let's look at this another way. I don't know about monthly costs - they vary so much with the weather that it's hard to tell. But the first two years we lived in this house we paid $4500 in oil to heat it, and the way I hear it we did OK because our neighbors paid more. Last September we ran out of oil in the tank and I said "we're done." We already had the boiler so I just went crazy stockpiling wood. I added an electric hot water heater partly for backup, partly because it was an easy way off the DHW coil in the oil boiler - I wasn't ready to plumb everything "right" to do the same with the wood boiler... not quite yet anyway.
I paid $1400 for wood because I didn't quite get enough put by and with this darn "polar vortex" I figure we'll still need another 2 cords at local market prices (currently $200-$225 per cord where "seasoned" means like 3 months, as near as I can figure by moisture content..."). And I'm still chuckling all the way. I could buy twice as much cut/split and still be ahead.
So I'm moving 12 cords around with a darn wheelbarrow and my neighbors can't figure out why I'm smiling all the time. But I know... And you all know...