I make my own diesel... I get used cooking oil delivered for 35p/liter, chemicals for about 8p/liter.
My over all cost per liter is about 45p per liter!
As the retail cost goes up, i expect my used oil will too but it seems to be in proportion.
It stings a little in the winter as I need ot mix 20% "normal" diesel in to prevent waxing and it feels expensive doing this!
iX