Shell Chemicals paid approximately $6 million less than computed fair market value for the former Horsehead Corp. property where a petrochemical plant may be built, a county official says.Shell paid $13.5 million for the former zinc smelter site along Route 18 in Potter Township, the Beaver County Times reported Thursday. The computed fair market value of the property is $19.6 million, said Janice Beall, Beaver County recorder of deeds.    Horsehead Corp. Senior Vice President Ali Alavi said Shell paid just under $17,000 per acre for the 797-acre site, which is "generally in the range of what an industrial property would go for."    Jack Manning, chairman of the Beaver County Chamber of Commerce economic development task force, said the sale price reflects that demolition and remediation have been ongoing at the site since 2014.    "I imagine that, after 60 or 70 years of a zinc smelter at that location, there was probably some very significant remediation that had to take place, and that lowered the cost somewhat," Manning said.    R.T. Walker, vice president of the National Energy Facilities Group for real estate firm CBRE, said both the company and the region benefit from the sale.    "It's very hard to gauge prices because they vary greatly up and down the river depending on where they are," Walker said. "But they did get a great deal, and it's a great deal for Beaver County and the region as a whole."    Shell has not made a final decision on whether to build its plant on the site.