A "normal" pay would probably have been closer to half a dollar to a dollar per day (if even that), 300 days a year or so. So we're talking $3000~$6000 total, tops.
This is basically a big straight trench, so you could probably have done it in a few days or weeks with an excavator. An excavator rental costs between 800 and 3000 a week depending on the excavator class (size and engine power, minis in the 15-40hp range will be at the lower end, the stuff you can see on construction sites will usually be in the 150hp range and closer to the higher third, I'm excluding the ridiculous stuff such as the CAT 6090 or the baggers which make sense in neither scale nor cost for this comparison)
So the purely monetary cost would probably be about the same if you had an excavator close-by which you could rent (and you were a skilled operator).
edit: actually for that kind of works you'd probably use a backhoe loader rather than a straight excavator, but the costs should be in the same range
edit 2: but to answer the question as asked, it would be way cheaper to use even a full-blown set of heavy earth-movers (and crew) than to pay the guy $10/h, 10h/day, 300 days/year for 22 years (which would cost $660k)