Lots of variables there. Get a few quotes... Its free
to do so and will you an idea of costs.
While the furnace is the cheapest part of the deal, the
duct work is going to cost some bucks.
I would also look into if your local utility companies offer
some rebates on converting systems. It may not be
much but every little bit helps.
Look into spending a high efficency unit too, a little more
money now but at todays fuel prices will payback real soon.