407 is worth $32 billion. Buying it is at least 5 times more expensive than building a new one.
As a matter of clarity, the government still owns #407; the consortium holds a very long term lease.
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But as
@wopchop notes the government has a variety of options other than buying into the lease, or buying out the lease.
The government could choose to impose tolls on #401 which would drive traffic to the #407, thus increasing its revenue and value. Perhaps the consortium could pay the Province to toll its competitor.
Either in the form of a dividend/one-time payment OR in the form of lower tolls.
The government could also offer the right to the contract to install and operate the tolls on all other 400-series highways.
In exchange for lower tolls on 407; and an equity transfer OR shortening of the lease of the highway.
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In the alternative (though I would oppose this) the government could build 413; even extend it east to 404 and make it free; that would undermine the #407's business model.
In exchange for not doing that, #407 could be asked to be more accommodating.
Needless to say, there's lots of leverage there for the government to use, should they be so inclined.