Using the actual power consumption of my Hayward Powerflo Matrix 1.5HP 2 speed and 250lb sand filter :
at 14 cents per kwh (not sure if this is the actual cost at this time or not but close enough)
24/7 on low speed (171 watts) which is what I do : $17.50 per month
10 hours per day on high speed (890 watts) : $38 per month.
I don't know how much water it moves vs high speed but it's more than enough and I'd even say it skims better in low because the water slows down so the skimmer pulls in more off of the surface.
From what I can tell, this system uses 19% on low speed.