What is the cost of a transit visa to China for an Australian having 2 entries within 3 to 6 months

Dear David, visa fee is AUD 90 for a double entry, but I am not sure where are you going to apply for the visa. If you reside within consular jurisdiction of Chinese consulate in Sydney, Melbourne, or Brisbane, you need to apply through CVASC, the Centre will charge extra AUD 38.5 for their service.
BTW, please be noted that visa is not required if the transit is less than 24 hours without leaving the airport. If transit in Shanghai, visa is not required for Australian passport holders for 48 hours and you can leave the airport during stopover.