United Airlines offers roundtrip flights to Mexico and the Caribbean from select cities in the U.S. with prices starting at $303.20, as provided by ShermansTravel. That's the lowest price we could find for select routes by $10. This price is valid on flights departing from Washington D.C. (IAD) to San Juan, Puerto Rico, (SJU) on October 5; prices vary depending on the departure cities and dates chosen. Book this travel deal by July 31 for travel September 1 through October 31.