1988 Honda Accord Value?? Help!?

I've been driving my grandpa's 1988 Honda Accord for the last several years and now I'm getting ready to buy it from him. He said to pay him whatever I feel like. I want to be fair and not take advantage of his kindness but I don't want to overpay either! The car runs fine (had to take it in a few times last year) and has a fairly big dent in the front bumper from an old accident, but other than that it's a great car. Advice???