Yeah - fuel prices have a huge impact on payback period.
I ran the numbers with higher Canadan fuel prices, Canadian purchase prices ($2500 difference), the more realistic "combined" mileage (who drives just highway?) and the Micra holds the lead as most economical until the 8th year of ownership when the Mirage takes over -- that's at 160,000km or 100k miles. That's for the manual. The automatic Mirage surpasses the automatic Micra 3 years sooner.
Full details/assumptions and link to spreadsheet:
Micra running costs comparison vs. competitors
And if you keep the Prius C long enough, it takes the lead from the Mirage... Somewhere around year 18.
Also: it's a reasonable bet that Mitsu & GM are going to respond with cash on the hood to close the price gap when the Micra hits dealerships next month. So that'll change the math again.