Depends how bad you want it, how nice it is, how far it is, how many miles, which model, etc...
That is a pretty vague question, but I would say bid what you are comfortable paying for it. You can never go wrong with that method. If you're comfortable paying more than the next guy, you might win and you'll always be happy. If you lose it and you're not OK with that because you would have gone another X dollars to get it, then you didn't bid what you were comfortable with.
If you are looking for pricing guidance, I would say do some more homework and inspect plenty of trucks before you bid hard and watch what they bring. That way you have an idea of where you should be.