Well it really depends on what the .com is, if it was for a US city, then id say not worth that much in .com.au form. Though if it was a generic keyword, you would probably have to look at how big the market is in Australia compared to the US to see just how much potential for customers/visitors you have. I dont think you can just put a % figure on how much a .com.au is worth compared to the .com domain. A domain is really only worth as much as someone is willing to pay for it. Just like anything really.
if perth.com sold for $100,000 id say that perth.com.au would sell for a bit more than the 10% - $10,000, Id say that it would nearly go for as much as $100,000 also. But it really depends on how well the buyer has done their research and worked out how they will get a return on their investment. Im not that experienced enough to be able to put a price on high dollar domains.
i said perth , although melbourne also sold the same week for $700.000 but there is also a melbourne is america , yes perth.com.au gotta wrth some good mony , it's one of only 7 capitol cities or 8 if you include canberra