The sweet spot between 'cheap' and 'quality' is...just good old inexpensive. Fly in style when you book through Flight Network, and don't worry about the cost from Sydney to Mount Gambier. You're covered by their Price Drop Protection.
Flight prices include taxes and fees and are based on payment by Visa Debit card which does not attract a fee. When completing your booking a payment processing fee of 1.8% for MasterCard and Visa and 3.5% for American Express applies. Note a fare is subject to change until a ticket is issued.