I figured she simply didn't remember what the copy Sotheby sold went for, and didn't want to sound stupid by guessing. (I didn't remember either, but it was a $12 million hammer price + 20% buyer's premium commission for a total of $14.2 million.) It's an unanswerable question, anyway - only 11 copies are known to exist, and the last one sold on the open market before this one was in the 1940s. You can't frame a market appraisal, even an off-the-cuff one, for something that gets sold once a century. A more interesting anecdote might be how the namesake and founder of her museum (A.S.W. Rosenbach) swindled their copy out of an English gentleman in the 1930s by insisting the seller name his own price for the book without giving him any indication that Rosenbach knew the real value of what was being offered. He paid what would be in today's money about $15,000.