What is the cheapest form of commuting ?
With my electric bicycle, I commute 54km round trip with 460Wh of energy, or 8.5Wh/km . This energy usage is equivalent to 0.052 litres of fuel, or 0.1 litre/100km .
Well, hangon, surely it is cheaper to pedal with your own muscle, right? That's what I thought as well. So, being a skeptic, I did my own calculation:
Assume the rider is from Tour de France (NOT me), which can 'easily' produces 300 Watt all the time in the commute duration (to achieve the same comparison with my electric bike commuting). Let say the rider manage to output 460Wh exerted at the same time duration (1 hour and 40 minutes round trip). The rider would need 400 Calories ! Quick googling tells me, this is equivalent to a bigmac .
For food cost, this translates to $5 for a round trip. Heck, that is the same cost (fuel only) if I use my Toyota Corolla! There you go. Human powered commuting is NOT cheap ! (Of course, I'm ignoring the health benefit here).
My electric bike fuel cost? ... Nothing! That's right, because mine is solar powered (one way). The other way, I charge at work for free :)
"Wait a minute, you have not included your capital cost!" I hear someone complaining. Well, since I'm a cheapskate, my total electric bicycle cost is around AUD1,600. This price includes: new electric bike kit, second hand bike, second hand solar panel, and my own custom MPTT charger . To date, I've clocked 11,000km and 420 cycles of charging (quite deep too) and definitely still have my 80% capacity (I haven't had the guts to test the actual remaining capacity). I predict I'll be good for at least another 400 cycles (before I need to buy a new battery), so the life-cycle cost would be AUD1,600 / 800 trips = 2 bucks a trip (or 7.5 cents per km). Try to beat that!
 Exact value is highly dependent on wind. This figure is anywhere between 350 to 550Wh. The average speed for the whole ride is between 33 to 34km/h (not much affected by wind, almost none I say). Data source from installed Cycle Analyst on-board (http://www.ebikes.ca/product-info/cycle-analyst.html)
 A litre of fuel contains 8.9kWh of energy, using data from http://www.afdc.energy.gov/fuels/fuel_comparison_chart.pdf
 460Wh = (460Watt)(3600seconds) = 1.656MJ = 396kcal (or 396 food Calorie, yes the Calorie unit IS confusing)
 Assuming 100% efficiency converting those bigmac calories to pedal energy.
 More reading if you don't trust me: http://www.fao.org/docrep/010/ah810e/AH810E08.htm
 For smart a$$ out there that says "cheapest commuting is no commuting at all, i.e. work at home!". To that, I can't top it off. Yes, I agree with you.