Good luck with opening up this can of worms guys,
Not sure in other States but in NSW all mods have to be certified by a licensed certifier no longer an engineer.
And the reason they charge so much is probably 2 do with this
"As a certifier, you need to hold $20 million public liability insurance and $5 million professional indemnity insurance"
And the fact there is no set charge, its just negotiated between certifiers and their customers.
Links for NSW
Modified and non-standard vehicles
http://www.rms.nsw.gov.au/roads/regi...nstandard.html
Vehicle Safety Compliance Certification Scheme (VSCCS)
http://www.rms.nsw.gov.au/business-i...ccs/index.html