Last fall when I installed my boiler I had nearly $12K in the entire system and I figured at that point it would have a pay back of about 3 years. I was spending nearly $3500 in Propane a year so I kinda hadged on the pricing staying even or increasing as it had over the last eight years. So needless to say it made complete sense for me to put the system in, but I also do not have to pay for my firewood nor do I have a bad back (YET!). I guess I would have to think harder about it if either was the case but for me...... I burn wood!
Oh as a side note.... If you have a large enough heat exchanger and furnace to push the hot water needed, the forced air can be very very nice!!! Compared to my propane heat.