Indirectly other countries do a similar thing; UK government makes you pay for the cost of shipping your application back to the UK to renew your passport (which is implicitly an overseas surcharge).
OZ is a country known for trickery in my experience! Is this worth paying that much for occasional travel - I might travel twice in a 10yr period
Only you can answer that question.
I might go on US passport and ETA might cost me $15? I heard of dual citizens got ETA approved.
Unless your passports are under different names, it won't happen. When you apply for ETA, if you're an Australian citizen, the screen will stop you mid-application and say "You are an Australian citizen." Won't be able to get it.
See
this link from DFAT.
Last time the fee increase was in March 2014 to US $218.
Australian passports are very expensive these days. Nothing you can do about it.
As an Australian passport holder, he/she can travel on that and won't have a problem getting back into Australia.
Oh, and in response to your subsequent point:
third world countries charge money from dual citizenship holders to keep the citizenship from the country of birth. I now realized that down-under is also one!
If you define the cost of renewing a passport as "keep the citizenship from the country of birth," then you're wrong. Renewing your passport does not 'keep your citizenship.' You're a citizen until you renounce it.