Glockaholic,
I have the latest (29th) edition of the Blue Book in front of me right now. If you want to post what the gun is within the next 1/2 hour (before I go to bed), I'll post back the values listed in the book for the various percentage grades of whatever gun it is you have.
As others have said, the book isn't really the last word, but it's a good starting point. Also, you probably want to be pretty confident in your ability to judge the gun's percent condition accurately.
Anyway, let me know.