To be honest, I don't notice it on my bill. It must have an impact, but .... if I was to hazard a guess I'd think it's similar to running one of those window-unit air-conditioners.
I don't run mine except when it's very cold outside. With the weather we've had since November, I haven't needed to use it at all this fall/winter (except to dry up carpet when I do another basement flooding). It took out 12 gallons of water from my carpet last week (I left my RO unit on, not noticing that the waste water line had been knocked out of the washing-machine drain line...
ops: )
It really is more of a localized effect. My house still sits at an overall 60-70% humidity even when that thing is cranked.