We’ve finally used our new PHEV in colder weather, so I have some rough calculations as they apply to us here in Colorado where it’s cold, but not, forgive me stupid cold. 
During the summer, with mild A/C use we were running around 3.6 miles kWh.
During the last few weeks, with lows around 20F during the morning commute and around 40F during the evening commute, it was more like 3.2 miles kWh.
Granted, these are approximate, and my wife is a pretty conservative (read, she doesn’t speed at all) driver, but it does include a mix of Interstate and in-town driving. At our current kWh costs of approx $0.12 per, we’re doing pretty good.
Comparing costs, driving in HV mode we get about 38 mpg combined, so at $3.12 a gallon for mid grade that’s a little over $0.08 per mile driven. So in all electric, even in the cooler temps, roughly half that. Now, I expect to loose another .2-3 miles per kWh during the really cold days, where the highs are in the single digits F, of which I can normally expect 4-5 over the whole season, but for obvious reasons we try NOT to go out duing those days.
[ note, mid grade here in Colorado Springs is the expected 87 octane that Toyota requires for the vehicle, which would be ‘regular’ in lower elevations, “regular” 85 here would be 2.73, or a bit over $0.07 per mile driven ]
And as it’s a PHEV, we do have the advantage that if it is cold enough, you can force the ICE engine on for additional heating (waste and otherwise).