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The difference between these two totals ($919 million minus $461 million) is $458 million. That number has been rounded to create the $500-million savings.

But it is a meaningless number. Nobody goes into a transaction adding up 100 years worth of costs measured in today’s dollars. That would be like looking at a $500,000 house and then adding in all the costs of maintaining it over 50 years — two roofings, a new set of windows, two kitchen refits, two bathroom renos, total redecoration of all rooms at mid-point, two basement renovations — and concluding that the price of the house is actually $1.2 million.

Nobody would do this because it would be stupid. The better way to calculate the long-term cost of projects is to estimate the “net present value” of all the future expenses. As the table nearby shows, the main reason the Hybrid option balloons to $919 million is the $505 million cost of maintaining the Gardiner over 100 years.

But that $505 million will not be spent for decades. City officials have not released the details, but they have said that future maintenance includes girder recoating in 10 years, deck concrete surface patch repair and waterproofing reinstatement in 35 years, and deck overlay in 65 years.

How much would the city have to put away today to pay for these and other costs over 100 years? That question, a crude version of the economic issue, is what city staff answered when they used “net present value” analysis to determine the future cost of operating the Hybrid Gardiner over 100 years. The net present value of $505 million over 100 years: $76 million, described by city staff as “the most reliable measure” decision-makers should use.