I'll take a stab at this one.

Let's say your pump pulls 10a@220v. That's 2200w.

2200w*720hr in a month=1584000 watt/hours, or 1584kwh per month.

I think electricity runs anywhere from .07 to .10 per KWH, so we'll take the .10. So that should put you at ballpark $158 a month to run your pump.

Adjust for your actual pump amps and electricity cost per kw, of course! If either number is substantially lower than the hypothetical numbers I used, your monthly cost could be much lower.

Shrimp