What do I add to the price for the car with a pl

A plow might be something a buyer wants, but in most cases, they don't. Plus that show that the truck was driven hard, which might dissuade buyers. I would price the truck normally as if there wasn't a plow. You won't recoup any money for it. You might want to remove it and sell it separately?

The plow might add 1/4 of the price of a new plow but as Dolby says the truck has probably been used and abused.

Exactly, especially if the vehicle is in a region that gets a lot of snow. It's along the same lines as my Father would never buy a car with a trailer hitch on it, a used station wagon, or a used pickup truck. He said those vehicles were driven hard and overloaded on a regular basis. Hope that helps!! Jim