SANTO DOMINGO, Dominican Republic — Investors from the United States believe they have found an exotic new prospect: Latin American baseball players, some as young as 13 and many from impoverished families. Recognizing that major league teams are offering multimillion-dollar contracts to some teenage prospects, the investors are either financing upstart Dominican trainers, known as buscones, or building their own academies. In exchange, the investors are guaranteed significant returns — sometimes as much as 50 percent of their players’ bonuses — when they sign with major league teams. Agents in the United States typically receive 5 percent.