Dear Donald,

Thank you for your help and patience.

The presented object appears to be a fossilized sort of ammonite without a riffled surface with measurements of 14" by 14" by 4.5". This Mollusca fossil is often found petrification and the winding of the shell is good visible preserved. A paper label names "Exogyra ponderosa Coemer" that is similar in the structure of its shell to nowadays Oysters.
A fair market estimation would be between 100 to 250 USD.
This estimate is based on actual recent past recorded auction sales of comparable items. Retail "asking prices" can be higher and may vary.

With kind regards,
George