free wood is never free..(i know you mean no cost) DD i am guessing from your description you will be burning (depending on your type of wood) somehwere between 8 and 11 bush cords (real cords 4 x 4 x
a season that is about 24 to 33 cords of what you see for sale in the paper (face cords)
from your post you are likley spending near 1700 to 2000 a year for your fuel bill now
i am not sure of what a new OWB costs now but i will guess you are handy and can do a lot of the hookup yourself so lets say 8 to 10 grand
that would be about 5 years to payback (at todays price and you know they are going up)
that is also not counting any money for fuel to get wood or machinery or your time (i never consider that because most folks have the equipment on hand and time is worthless if it is not time away from a real paying job. if you cut wood instead of watch ing TV it is better for you anyway
i think they are worth it but you may not..also remember you will be married to this thing, it needs wood likely twice a day no matter how you feel or where you are . this will be a major investment, think it through good before spending the money, make sure you are ready for the commitment.
good luck on however you proceed