I seem to think this came up a few months back, but it seems to be back in the news again - a full page article in today's Guardian, no less. Cormac McCarthy is auctioning his typewriter which he has used since the 1960s. "I have typed on this typewriter every book I have written including three not published. Including all drafts and correspondence, I would put this at about 5m words over a period of 50 years," he is reported as saying.
The apostrophe key, of course, is practically unused.
I have to say, I think the estimate of $15,000 - $20,000 seems a bit low to me. I expect it will go for much more.
[UPDATE: It sold for $254,000... That's an expensive typewriter...]