Elvis item...how much is it worth?

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Help me guys. A very old friend gave me something before he died. He told me this would be my pot of gold. I have a photo negative (not the photo) of Elvis Presley. It was taken in the 50's at what I believe was a county fair, and is talking to fans. It is a beautiful thing to see, and would make a wonderful portrait. Really, it's that good. Now, my friend told me that only ONE photo was ever printed off that negative, and that photo was destroyed in a house fire in the early 70's. The negative is the only thing left of this memory. So with that making this an unpublished, never circulated photograph of The King, what would it be worth? I had an offer of $1,000 from a memorabilia dealer, although he told me it was worth much more. That was all he could afford to offer. I refused, of course. But I really want to have a realistic idea of what this negative could really be worth. Anyone???? Please tell me how to find out. I am anxious.