Due to multiple factors, expect to potentially pay more when booking in July. For low rates, April can be a great choice for travel to Duluth. Flights to Duluth from Rapid City in April are about $337 on average during April, but can be found for as low as $176.
Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Rapid City to Duluth as you normally would.