value of signed sports memorabillia

I was wondering what the value of Matt Holidays 2008 750 LI BMW that I purchased would be. Since I purchased it, I have gotten him to sign the back of both headrest of the front leather seats. I have a copy of the title with he and his wifes name on it and pictures of him in my car signing the headrest. I dont know where to go to find out the value of something such as this. It has just a little over 20k miles on it.

You need a professional-certified auto /motorcycle appraisor to help you with that. Make sure they are certified. You should be able to find some one local on the Internet or yellow pages .
Hopes this helps.