How much is it worth?

[Singh]

Someone wants to do a deal on my car but wants to part exchange a 4 speed 1996 A8 4.2 quattro sport. Green with ecru interior. It has been really well looked after with 5k spent on it in the past two years along with another gearbox from A8 Parts. It's done 206,000 miles. If I took it in part exchange I would only want to sell it on asap. How much should I offer the potential buyer? He sounds like an enthusiast so I would love for my car to go to him.
Not too interested in guide prices as I think they are fairly irrelevant when pricing these cars and I don't want to insult him with a £500 offer a la glasses guide.