In Chicago we have had a fairly brutal winter for us; My mileage has dropped form 32/33mpg during the summer to 26/27mpg during the winter. If you look around, there are a lot of people who don't get summer mileage about about 27/28, so I'd expect their winter mileage to be as low as 22/23 mpg.

One issue that will make a considerable difference is whether you will be able to purchase and use a block heater. That combined with a front grill cover, to reduce cold air entering engine compartment, could give you around 50% or more of the mileage you would loose to cold weather back. That could make the car more practical.