We will be visiting Beijing for 5 days and Hong Kong for 2 days. What is the visa cost ?

You don't need a visa to travel to Hong Kong. Just make sure your Australia passport will be valid for at least 120 days on the day you enter Hong Kong.
The requirements for a China visa include: completed visa application form V2011A; your passport with at least 6-month validity and at least 2 blank visa pages; copies of photo page of your current passport as well as other ID, e.g. previous passport, driver license, etc.
If you live in Queensland, you can deliver the visa application documents to CVASC in Brisbane to obtain a visa. Address: Part Level 4, 140 Ann Street, Brisbane, QLD 4000, Australia.
The total cost is $60 (single-entry visa fee) $55 (Postal application Service fee)= $115.
BTW, a single-entry visa is usually valid for 3 months since the issuing date. So you'd better apply for it 1-2 months ahead of your departure date.
FYI: http://www.visaforchina.org/BNE_EN/applybypost/index.shtml