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.

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

...wearing camo and got ammo...