The cost difference between using well-water (pumping cost) and municipal water appears to be quite significant in this area - about 26 times more expensive.
The irrigation rate for electricity is $3.54 cents per kWh. For a 1HP pump (roughly 1kW) pumping at 20GPM (under average pressure) this results in about 1200 gallons/hour, which means a cost of 0.00295 cents/gallon.
Municipal water at the farm irrigation rate is approximately 20.5 cents per cubic meter. One cubic meter is approximately 264 gallons. This translates to 0.0777 cents per gallon.