One of the rarest Windauer Silvertips that I've come across was on the auction site a few weeks ago. It was #13 of 25 commerative bows done by Dave in Pauls memory. The first time it was listed it went over 1500 and had not met the reserve, the second time it was over 1400 and again did not meet reserve price.
Other than that, I would think that any of Pauls bows done in exotics would be rare as most that I've seen had been built to be workers and not necessarily lookers. Mostly diamondwood (actionwood ?)
Another guess would be that bows made for his close friends like Bart Steyler would bring a higher price.
Tim