Due to multiple factors, expect to potentially pay more when booking in September. For low rates, April can be a great choice for travel to San Diego. Flights to San Diego from Riviera Maya in April are about $368 on average during April, but can be found for as low as $195.
Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Riviera Maya to San Diego as you normally would.