What will it cost less running 460V motor or 208V motor?

No, you are charged by the wattage that you use. Watts are the product of amps times volts. Say you have a device that draws 5000 watts and you are using a voltage of 480 volts then the amperage will be A = W/E = 5000/480 = 10.4 amps. Now using that same wattage at 208 volts the amperage will be A = W/E = 5000/208 = 24 amps. As you can see the only thing that changes is the current (amps) on a consistent wattage with different voltages. One big advantage on using higher voltages over smaller voltages is, with the amps being lower, a smaller wire size can be used for the same wattage load. In the end though, you are still billed on the wattage used.