I use a custom checkbox to track autographed issues and a custom text box to record details about that issue (when and where it was signed, etc.). I think the volatility of a signed book's value is too dynamic to maintain in the master database. There are several contributing factors that make it very difficult to adequately assess the value of such a book. Greg already mentioned some of these. Some others include the perception by many collectors that an autograph changes the "mint" status of a book because it's no longer in its pristine, unaltered condition. Another is the placement of an autograph, e.g., on the front cover, inside the cover, on the splash page, or somewhere else.
r/MIke