It would be worth about $4500 if it was in pristine condition. Nice aftermarket accessories like quality cans or a PCIII would make it more appealing, but I still wouldn't pay anything over that for it as you can get a new one for $6500-$6900 if you know where to look.
I'd guess you might be near the end of chain length, might be able to haggle the price down a little based on that. Also, if it hasn't had the valves done yet (16,000 mile maintenance), you might be able to get the price down a little for that, too.
The valve thing isn't absolutely vital, from what I hear. Seems most 919 owners who have had it done found that the valves were still in spec. All the same, can't hurt to have a haggle point. I just like saying haggle.
Oh, and I'm with LDH on the price. 4500 is probably a good starting point. I got my '03 for 5900 with a paltry 6027 miles on it in absolutely pristine condition and w/ a spare gas tank. I might have slightly overpaid, but mine was immaculate when I bought it.