While my neighbors average gas heating bills are about $400/mo I average @ $65/mo during the winter for cooking gas and our clothes dryer. My furnace is typically for 'back up' only. I do use the cold air return and fan from the furnace (which is in the same room as the woodburner) to circulate the heat from the woodburner throughout the house without the burners in the furnace kicking on.
I go through @ 3 cords of wood in the winter heating between Dec 1st and the end of February. I get the wood for free (sans the work we do to in cutting down the dead trees on our property and hauling it 350 miles home) from family in the Upper Penninsula.
If you go with a fireplace insert, you will need a stainless steel chimney insert, that'll run you anywhere from $300 - $600 depending on the height of your chimney. Assuming you're paying for wood, consider how much you'll use the insert to provide actual heat. The economics of having an insert and heating with wood works for me only because I get the wood for free.
BTW I prefer the heat from the woodburner, it goes through my arthritic bones, something the gas furnace can't do.
The most important thing for me is to have it warm during power outages. We have survived for three days in the winter with the fireplace (because we have doors around our living room to keep the heat in, plust
hot water bottles :) ), but were a bit uncomfortable by the third day.