We at FVB  look upon a gallon of gas this way:

One gallon of gas equals 125,000 BTU.

3,400 BTU equals 1 KWH.

One gallon of gas equals 37 KWH (125,000 / 3,400 = 36.76 KWH).

Human work output in agriculture is 0.74 KW/Hour

One gallon of gas equals 50 hours human work  (37 KWH /  0.74 KWH = 50 hours).

[Source: Dept. of Energy]

There is an Acre and a Quarter of sod that needs turning to the next crop. An acre and a quarter equals 54,450 square feet.

Now, were you to turn that sod with your muscle and its multiplication of force by way of an implement named the broad fork and doing so at a rate of 30 minutes per 5 x 20 ft. bed (100 square feet) would require 16,335 minutes or 272 hours (54,450 / 100 = 544.5 units x 30 minutes/ unit = 16,335). Then, 272 hours labor expressed as human KWH =  201 KWH (272 x 0.74) As one gallon of gas equals 37 KWH our muscles have burned the equivalent of 5.43 gallons of gas (201/37 = 5.43). If the cost of the gasoline is $3.55/gl. the value of your work in these terms is $19.27 (5.43 x 3.55).

However, were you to turn that sod by agency of a 90 HP tractor and 10 foot chisel plow operating at 1495 RPM and with gas consumption of about 0.60 gallon over the half hour of the tractor’s operation upon those 54,450 square feet you would have expended a whole $2.13 in raw terms ($3.55/gl. x 0.60 hrs. = 2.13) .

We do have the expense of the tractor and plow. As in their purchase, maintenance, depreciation and such and running about $54.00 for every hour of operation. So, gas and other costs of the tractor and plow total $29.13 (54/2 = 27+2.13)  Now, let us posit a wage, an expense at that, for the 272 hours of human toil. Let’s pay the crew 10.00 dollars an hour.  With said remuneration times the  hours worked  we have paid out $2,720.00 (272 x 10). 2720 : 29.13  Or, about 94 :1 Well, at that ratio, gotta' love the smell of gasoline in the morning.

I’m in a vision Year 2016 on a Sunday drive touring Wisconsin's Kettle Moraine. At a corner convenience station I see a  Sign and it reads: Gasoline /$62.97/gl.  Cheap!