Two sites ? one on Chula Vista's bayfront and the other on rolling open land on the city's east side ? are emerging as the leading candidates for a Chargers stadium in South County that could cost as much as $1 billion. The Chargers paid $220,000 to the design firm Cooper, Robertson & Partners to rank four locations in Chula Vista. The firm, based in New York, evaluated size, availability, parking potential, transportation access, environmental concerns and commercial development prospects.