So should the major cities have a rental allowance for players under a certain salary (say 60K) in addition to the cap?
No. They already have other factors that balance that. Would you rather be an import playing in Sydney or in Townsville, for nightlife, for example?
Until recently, Perth was the most expensive city in Australia to live (most expensive in the world according to one survey.)
And I can assure you, nobody comes to Perth for the nightlife.
That said, it has been my experience that living in regional cities is often more expensive than capitals.