I designed and built my house around the use of an outdoor wood boiler, so technically it hasn't saved me a dime. However.... the background is that our old house, which is half the square footage of the current house, used anywhere from 900-1100 gallons of fuel oil every winter, depending on the weather. Now, the new house is more energy efficient, but it's much larger, so there's a small trade off. Let's assume that the new house might use 25% more fuel than the old one. Probably a bit too conservative but whatever. So 1000 gallons of #2 oil at $2.50 a gallon (again a reasonable assumption, based on historical prices in my area) x 1.25 = $3,125 per year. This is probably a bit low.
Now, the old house was simple. Automatic delivery, no hassle, pay the bill. No problem (well, except for that "pay the bill" part...)
Today it's different. Outdoor wood boiler means equipment for wood processing. I don't buy wood, I harvest it off my own land and my parent's land, so there's no cost for me for the actual wood. However, this has necessitated the purchase of a fair amount of equipment - a tractor and bucket loader, a log splitter, several chainsaws, wood and log handling tools (i.e cant hooks, log tongs, maintenance stuff etc.). All told I've probably invested $15,000 in the equipment (although I use it for a lot of other things besides firewood). The initial investment for the first wood boiler I owned was not counted because every house needs a heating system, so it was either spend it on a OWB or spend it on a gas or oil furnace. The second wood boiler (my current one) was a $10,000 investment to replace the first one. So let's say I've spent $25,000 over 12 years on wood heat. That's about $2,100 annually. If I had to buy wood, I'd get it in tree length delivered to my back yard and probably not need the tractor (but I'd likely still have some kind of ATV or UTV to do other things), I'd also be out another thousand bucks a year for purchasing the truckload of wood. So, if I had to buy wood all this time, it probably wouldn't have saved me any money and I would have saved countless hours spent busting my ass outside in all kinds of weather, processing wood and feeding the OWB.
However, I still feel I've saved a lot of money over the years, because I have a really hard time writing that check to the oil company and I would rather spend 2-3 weeks worth of time every year gathering firewood. Now that I have a new OWB and all the rest of my equipment is in good shape, now it's just the annual value of the BTU's harvested, measured against the depreciation of the equipment. I tell my wife that I save so much money burning wood that she ought to let me buy a new chainsaw every year, but so far it's not worked, I think she's smarter than that.
In ten years maybe it'll be time for a new tractor or a UTV or something like that.