Even with petroleum diesel at $4 per gallon, farm-based renewable fuel can't compete on price. By Marianne Lavelle. + More. The retail cost of highway diesel fuel is $3.99 per gallon—thanks to tough environmental rules and strong global demand, especially in Europe.The national average retail price of diesel hit an all-time high for five weeks in a row, is above $4 per gallon in plenty of places, and is up 50 percent over one year ago.he national average retail price of diesel hit an all-time high for five weeks in a row, is above $4 per gallon in plenty of places, and is up 50 percent over one year ago.