Following years of frustrating efforts to come up with a price for a large section of land at the old South Weymouth Naval Air Station, the developers of SouthField signed a binding deal Tuesday to buy the land from the Navy. In August, both sides reached a preliminary deal for South Shore Tri-Town Development Corporation to buy about 680 acres of land from the Navy for $25 million. They had only until Tuesday to sign a binding purchase and sale agreement. Tri-Town, which is overseeing the redevelopment at the base, and LNR Property Corporation, plan to build 2,800 homes and develop commercial space, as well. Construction has already started on a parcel of land the Navy had transferred to Tri-Town in 2003.