Cheapest per hour in the air?
Cheapest per mile over the ground?
Cheapest per passenger mile?
Flying need not be expensive - I bought my Archer last year expecting a cost in the B$300-400/hr range for 50-75/hrs per year - one thing was a fear that I wouldn't fly it enough and it would have maintenance issues as a result, so decided to let others rent it, expecting an hour or two a month.
Turns out I can rent it for closer to 150hrs per year at B$400 - it generates a profit that offsets my cost - even with my five figure insurance premium engine fund, hefty remote location annual costs (shipping parts, duties, customs broker fees etc) and $15/USG fuel it costs very little - I'll not say how much per hour (you'll not belive it), but it's less per hour than I paid for a frothy coffee last time I was in London.
According to my sensitivty analysis when I cut the hourly rate in the summer, my flying will become cheaper with greater utilisation - would like to get to 350/hrs per year.
The cheapest airplane is the one that is commercially utilised (in my case I am in the relatively unique position of being a virtually monopoly position (one of only three four place singles available for rent in the country - there are only six GA single airplanes, one's a microlight and two haven't flown for a couple fo years) came as a surprise to me - so I too can say that the difference between what I expected it to cost and what it actually cost are hundreds of dollars apart.