What is the value of a broken bat autographed by Kelly Gruber?

Because you are asking the value of a Signed "Broken bat" I'm going to take it that you have a "game used" bat, used and signed by Kelly Gruber. A broken store model bat to display a signature will have very little value. The bulk of the value will rely on The provenance or authenticity accompanying the "Game Used Bat.

With out any letters of authenticity as a game used bat, with the proper bat markings as a pro bat it will only sell as such. A Kelly Gruber "broken" pro model bat. As only a pro model bat it will still have a low collectors value because it is broken. A Kelly Gruber Signed "Broken" Game used bat would probably sell for about $75. -$125.,and less without a COA for both the Game used bat and signature.

Keep in mind that most "Game used" collectors would rather have the item unsigned, and autograph collectors are looking for a displayable signature