I know that George Washington, Napoleon, the Duke of Wellington, were prolific letter-writers. As such, shouldn't their materials be on the lower side?
It is an interesting question. I have not followed historical autograph sales much since I changed my collection to classic entertainment many years ago. I believe there will always be a ready market for Washington, regardless of the number of autographs available in the public domain. The internet auctions have played a role (in my opinion) in inflating prices. People log on to eBay and see a document signed by him for $20,000 to $ 50,000 and believe the one they have is worth that. It just depends on how long it sits at that price. It would be an interesting study to examine the actual sales made versus the asking prices. Significant historical figures appear to have kept their value steadily over the years. My observation, albeit not particularly valuable, is that I do not think any of those autographs will decrease significantly in the foreseeable future.