You are charged for (kilo)watts and the watts stay the same no matter the voltage. Whether it be 120v x 4amp = 480 watts or 240v x 2 amps = 480 watts. The savings is only in two places:
1. If you are setting up a new circuit. Say you need 6 pumps that each draw 4amps @120v. 6 x 4amps = 24amps. 24amps will not cut it on a standard 20 amp circuit so you will either have to run larger conduit and wire (10 or 8ga) to upgrade to a 30amp circuit or you will have to split the circuit and maybe double the amount of 12ga wire, conduit, etc. If you change the voltage of the pumps to 240v you have effectively decreased your amps on the circuit: 6 x 2 amps = 12amps. Now you can run them on standard 12ga wire with no more wire than you would have needed for a standard 20 amp circuit.
2. Electricity is like water in some ways. Resistance with electricity is like head loss with water. Longer runs create drag on the flow. The more the drag, the more it costs to run. The answer to the problem is again to upsize the wiring - or switch to 240 to reduce the drag.
Hope this helps.