PlayAsia.com is not in Australia, therefore not liable to Australian taxes. You cannot compare the prices. 2K Australia sets Australian pricing, 2K International sets Asian pricing.
As stated all the time, I have taken Mercade's prices. I did not check about whether any of the competitors is Australia-based or not.
Actually, since I am located in the European Union, this does not make much difference, as far as I can see, as the the - admittedly here will be a problem due to the verification - location of the sales transaction, which should be the location of the purchaser, is important. The latter is even emphasized by the fact that Mercator's statement even included shipping costs.
This issue may be handled differently in other locations on this blue planet, but that is something which I cannot find out - at least not within the limits of spending reasonalbe amounts of time and effort.
The difference in pricing could be quite simply one thing: retailer markup.
Steam adds 30% markup (this is their publicly known markup on games) whilst other retailers due to local competition have much less markup. I know some stores only markup games 5%.
HOWEVER, 2K Australia sets the price that retailers purchase the game from them for, and pays all the taxes, duties etc. This price to retailer includes all of this. So if they sell the game to retailers at $50 AUD, then Steam will charge $65 AUD (markup 30%) and other retailers will sell for their markup ($52.50 is 5% markup).
This here seems to be the vital part of the posting.
2K Australia sets a price for the retailers and the retailers (physical or virtual [internet-based] copy) determine the resulting price for the final customer.
Steam is responsible for the final price of the product (Civ5 in this case) being bought by making use of their so-called "service".
This is, what has been denied by the Steam Sunshine Squad many times.
Thanks for proving that my understanding of the matter has been correct.