Assuming that the WVO is free, and that methanol is somewhere around
$3/gallon, depending on your source of NaOH/Lye or KOH, I figured it
out to be between 70 and 90 cents per gallon. This is purely based on
the amount of methanol and KOH that is used per gallon of resulting
Biodiesel.
If you buy Lye at the hardware store, you will be towards the high end
of that range, if you get a 50 pound bag of KOH from a chemical
supplier, you will be on the low end.
Good luck,
Greg