About the same value as the guitar itself. IME, autographs (or, more accurately, marker pen scribbled afterthoughts) add very little to the value. IIRC, Metallica had an endorsement deal with ESP some ten years ago. Something must’ve turned sour because the boys very publicly disclosed how they got rid of the guitars and vented their spleen about ESP in general.

I’m not writing this to downplay your instrument. It’s just a fact that “signed” guitars are worth very little over and above that of the instrument itself. They’re rarely a goldmine unless somebody (to be blunt) famous and dead had signed it. Artists get asked to sign things all the time, and they’re cosequently not often all that rare.