How much does it cost to replace an internal combustion engine?
Without doing the math I suspect he spent more on replacing the batteries then gasoline would have cost him (and not counting the time to fill - perhaps 15 to 20 minutes compared to the time to recharge the batteries twice a day.)
At least from research, in my market, you can rebuild an IC engine for about $4,000.
I replaced a V-6 engine for $250...had less than 100 miles on it. Beats the heck out of replacing a electric motor.........