You know my boilers are my own design and maintained by me so they inherently are of very little long term operating cost outlay.
The new mini boiler I built this year was about 90% surplus metal and parts so maybe $100 to build it and about $300 more in setting it up at this point and I expect that once its going it will be like the old boiler and have a very low annual operating cost other than the typical half hour or so a week I spend on general maintenance cleaning out the flues and change out the paper towel based primary filter.
As for oil collection I do it for part of a day about 3 times a year netting me about 2000 - 3000 gallons a year now. So maybe 20 - 25 gallons of gas for the truck run around and transfer pump operation. Everything else is scrounged or long since paid for materials which I place near zero value on at this point.
Once everything with the new system is setup right and tuned properly I doubt that it will cost me much over $100 a year to heat my new work shed and old house combined. So given the present used oil consumption rate I am seeing it run at I expect it will burn about 1000 gallons a year or cost me about 10 cents a gallon or ~140K BTU or about $2.50 a MWH to heat my house.
Not too bad considering the same MWH on utility power would cost me over $120!