Before the federal government, or anyone else for that matter, gets too excited about the prospects of an airport link to Tullamarine as part of an overdue $1 billion infrastructure refund to Victorians, there's a far more humble figure that we need to consider. Seven dollars and ninety-six cents, to be precise.

The significance of this relatively meagre sum hasn't featured in the long-winded debate over the airport link but it should, because it represents a key factor in whether a Melbourne link will ever be built and, if so, whether it will ever attract the anticipated patronage.

Sydney's airport rail line is held up as a template for Melbourne and although Kingsford Smith is much closer to the city centre it shares with Tullamarine a critical characteristic: both are operated under 99-year leases from the federal government and their operators have the final say on what gets built on the land they control.

That's why it costs train commuters almost $8 to travel the one extra stop – less than 90 seconds of train time – from Mascot station, located outside Sydney airport, to the domestic terminal station located on the airport site.