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.
Restaurants have oil vats in the back alley...industrial sizes...500 gallons +...Hot summer day roll up with a hose, connect, drain, fill, bounce