An inscribed book is personal - to one person. Collectors don't want that type of item as much as they want just the signature. Of course, some collectors are really picky about that and others don't mind as much, so the value may have a wider range than does your typical called-for signed or just signed and not inscribed copy.
For example, I picked up a signed/inscribed first trade edition of Play the Piano... on ebay about two years ago for about $150 (more or less; well less than $200 certainly). Market price for a first trade edition of that title that is simply not called-for signed (not inscribed) would be about $200-$250 (there were 500 trade copies). The called-for signed edition, of which there were 300, would go for $250-$300. All of these prices are estimates; one look at ebay and you'll see why not much is moving these days.