BP Plc’s oil spill may drive down Gulf Coast property values by 10% for at least three years, according to a forecast by CoStar Group Inc. It was estimated in mid-June that losses could total $4.3 billion along the 600-mile stretch from the Louisiana bayous to Clearwater, Fla. The spill may cost BP almost $50 billion in cleanup and reimbursements for economic damage to the tourism and fishing industries. St. Joe Co., which owns 578,000 acres in northwest Florida, has seen its stock fall since April. The Company has built protective beams and taken aerial photos and soil samples as evidence in case it files damage claims with BP.