The short and annoying answer: we don't know yet. Anyone in the US who buys a PC running Windows 7 between now and mid-January will be able to purchase an upgrade to Windows 8 Pro for $US14.99. There's no price for Australia yet, but I'll be amazed if it isn't higher.
Microsoft says the Windows Upgrade Offer will be rolling out from June 1 in 131 countries worldwide. However, when I asked Microsoft Australia what was happening, the response was terse: "We will have more details to share at a later date".
I'm really hoping "a later date" turns out to be June 1 US time, rather than months down the line. If you're thinking of buying a new PC before the financial year ends, I'd definitely wait a bit longer to see if an upgrade offer appears. In the meantime, you can upgrade to the Windows 8 Release Preview for testing purposes.