Anyone know how to place a relatively accurate value on a fossil?

I picked this fossil of a few ferns at the flea market today for $4. The lady said she her uncle lived in Pennsylvania and found this. When I said I didn't know what it was worth, even if it was worth $4 she laughed and said yeah it's worth 20 times that. Think she is right? Beats me, but it's neat.

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Everybody's had to fight to be free...Tom Petty

...wearing camo and got ammo...