Do any of the results change if instead of picking a quite high margin in postholes, you pick something more reasonable? What if it takes 15 years to pay off the hole digging machine, not 6 months?
6 months worth of postholes is 200 * 30 * 6 = 36,000 postholes and a profit of $0.55 per posthole to pay down the capital investment.
In my original post I claim $5 per posthole profit can pay this off in 11 days.