I'm dismayed that people are suggesting closing Mascot! Why on earth would you remove such a useful piece of infrastructure? To do so and not face congestion at BC you'd need 3 parallel runways minimum - the site isn't even big enough for that. Longer ground transport times for anyone heading to east of Parramatta would be the result, and all for an 11 digit price tag, I would expect. At least 10 digits.
Oh, and I think you could come up with a more independent knocker than Sydney Airport. They really have no real incentive to lose their monopoly or expend a lot of money to retain it.
Sydney airport would tell you that they are able to cope with current demand. Consequently, I am imagining things when I've have to spend another 30 minutes waiting in an aircraft after a long haul flight waiting for an available gate. On other occasions I must have been hallucinating when the gate available at the time was actually on the freight apron and not at the terminal.
The inability of Mascot to service projected aviation demand, even given its master plan, is clearly laid out in the Joint Study. Those interested should read chapter 4 at http://www.infrastructure.gov.au/aviation/airport/western_sydney/sydney_av_cap/index.aspx
That joint study didn't explicitly consider relocation of the primary airport because, as it said...
This is in recognition of the economic and access benefits Sydney (Kingsford-Smith) Airport provides to Sydney, NSW and Australia as a whole, given its location next to the Sydney CBD, and its proximity to the market catchment for business, freight and leisure travel. Taking into account the existing sunk and programmed investment in infrastructure, it is expected the airport will continue to be the focus of demand particularly for peak business and high value air freight.
Scattered throughout the report, particularly in the appendices, are estimates of the cost components required for full airport relocation. I think you can comfortably say that you'd be looking spending at least $20 billion
in "before you can close the other one" expense to relocate and get an airport with equivalent utility - considering things like additional land acquisition for an ultimate three runway configuration, full length parallel dual runway construction from the get-go, the requirement for express transport links, construction of multiple terminals and engineering facilities.
Current spend for the secondary airport - airport + infrastructure over the next ten years - is running at about $6 billion.
The land value (not including the stuff you build on the land) of a 12 x 40 metre residential block in Mascot in 2012 was about $600 K - $1250 per square metre. The Sydney airport site, far from all of which would be "saleable" (i.e. - I'm being very conservative) , has an area of 907 ha. At $1250 per m2 that makes $ - giving a total land value of $11.3 billion. That doesn't close the gap. That's before we consider the benefits of competitive tension.