They are not very efficient. A lot of the BTU from burning wood goes up the flue.
Now a cost comparison between LP applince and wood fired appliance depends on the cost of each fuel source. If the wood is free than wood is cheaper to burn.
I pay 80 dollars for a cord and use 2 cords a winter. In my case it is much cheaper to burn wood than gas. I've seen some places that sell a cord for 200 dollars, it would be cheaper for me to run a L.P furnace if I paid this much.
On a L.P. furnace you do not "Have to fix the fire.", all you have to do is turn up the t-stat.
In order to determine cost between the 2 you need the cost of both fuels and the BTU output of both of the appliances.
If you have a good heat loss calc. and average temp of your area you can determine how much fuel you will use for the year. Do not forget the cost of the appliance's. The wood stoves around here go for alot of money (4 times the cost of a L.P. furnace)
That's what I was thinking. He uses the boiler heat when he is up there on weekends. He has an old oil fired furnace for when he is gone. I'm trying to talk him into an l.p. 90+. I think he pays $225 for a cord.