Everyone pays the same amount of money for this immigration.
I hold Bachelors of Engineering from UK which is on the list of exempted qualifications, I still did IQA and IPENZ...I even did EWRB registration as my case officer asked me to do so.
The same procedure was conducted for my wife as well because she is also a professional engineer.
Its ok, Immigration is always an expensive process, if some countries are cheap they charge additional money for partners being included...Like you said about Australia..For principal applicant there is fees and then for additional members you have additional fees...in INZ you dont have to pay anything additionally...
Requirements are requirements set by government of new zealand not by any individuals..