Biff Was Right About At Least One Thing.
He was getting screwed with his car repair bill.
$302 in 1955 would equal $3,257.87 today.
For a repair job that was strictly cosmetic and nothing mechanical, that would be way too much. Yes, I know it was full of horse shit, but once you got rid of all that (just take the car to wherever the truck was going) then it's mostly just a very throughout cleaning and detailing job which even today would not be $100. Being a convertible should somewhat help with the smell. And Biff mentioned a couple of dents, which would not have been an expensive repair. Even if the old man did charge $80 just to tow it away (also way too much), there's no way that bill comes out that high.
And if Biff really wanted to save some money, he could have driven it himself to get rid of the manure, then dropped it off to get fixed and cleaned.