*Do* the math, its very easy.
You take your usage before going to wood heat for the period and multiply it by the current rate charged by your supplier.
Prior to wood heat, my January average gas usage was 241 CCF.
Current rate of natural gas delivered is $1.485714/CCF
So at current rates, I would have paid $358.05 for January's gas usage
My usage this year was 35 CCF, my gas bill is $52.00. (no wonder they're sending someone to check my meter)
I saved $306.05 in January. People who use oil or propane would save more than me.
I gather all my own firewood, but almost never make a special trip so I don't really incur gas costs for my pickup outside loss of mileage carrying a load of wood home. I also need gas/oil/chains for my saw and gas/oil for my splitter.
Some would say 'what about your time?' but I enjoy it too much to consider it work. Gathering firewood is my 'gym time'. I get outside and excercise. I have a bad back and am fortunate to have a friend and coworker who lifts my wood into the pickup truck. I then roll the rounds right onto my splitter from the bed. I bring him split firewood in return.
Also, the cost/CCF above is a little skewed. My utility used to break out the fixed costs but they don't do that anymore. Those are your charges regardless of usage, so that cost/ccf would fall as the usage rises.