A dowser is less effective than a geologist and bears, at the minimum, a higher opportunity cost over the average (of instances of people searching for water with a dowser instead of a geologist).
A fine economic analysis, but you're forgetting the balance-of-costs comparison.
If what you saved using a dowser (who, by your own scenario, is cheaper than a geologist) is more than the cost of two wasted wells, the dowser was a cost-effective alternative. In that case.
If, on the other hand, the dowser wasn't much cheaper, or you had to sink 5 dry wells, or your dowser never finds water, the dowser was a net loss.
I think that on balance, the latter scenarios are more likely. If you're thinking about choosing dowsing, you're better off just throwing darts at a large map of your property and saving that cost for the same effectiveness.
But if you're going to do an economic analysis, show all your work.