That's pretty tough considering people's price for WVO can vary...
But I would argue against it being "free fuel" as John says, aside from a select few people that have access to mass quantities of that stuff. I have no idea where I'd get 15 gallons of used veggie oil on a weekly basis.
WVO can be had for free around here, but the problem is there just isn't much of it to go around. Boulder is hippie central and we have tons of aging hippies in rusted old mercedes, vw, toyotas etc pilfering oil wherever they can find it
h: If you could handle driving to Longmont to fill up, that would be one solution.