Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Illawara to Sioux Falls Regional (Jo Foss Field) as you normally would.
When travelling, it's generally smart to pack in preparation for rain or any other conditions that may arise. The rainfall in Sydney is roughly greater than that of Sioux Falls. You can assume this to be the case over the next 16 days.
Our data indicates that the weather will be 30 °C on average in the month of July. The temperature in Sioux Falls is typically hotter than in Sydney. If you're traveling when the temperature is at its warmest, push to plan early, as flight price and availability will have an impact on your travels. For the best results, search for flights well in advance or after peak season.