As of April 2018, approximate prices for good quality solar panel systems in Australia (Tier 1 panels + quality central inverter), fully installed, are:
3kW: $3,700 - $6,000
5kW: $5,000 - $9,000
10kW: $12,000 - $16,000
To be clear - the upper end prices are for top-end systems (e.g. LG/Sunpower panels with full panel level optimisation), installed by a solar 'craftsman'.
Note that the above prices also include the discount from the solar 'rebate'.
Costs can increase if you need a switchboard upgrade or other electrical work to make your home suitable for solar panels, or if the design of your home makes the install more difficult. If you decide to go with micro inverters over a central inverter, costs will also increase by approx 20-30%.
If you're quoted much less than these price ranges by a solar salesman, chances are they're cutting corners somewhere or using inferior quality products to deliver a lower overall cost. So, be wary!