Modern furnaces are 95% efficiency. They didn't actually have minimum standards until 1992, when it was 78%. Figure, what, 66%? On top of that efficiency drops over the life time. It'd guess a replacement furnace will cut your bill by at least third, with no telling what the upper bound is. If you do the energy audit thing they can at least give you a good estimate on what the return-on-investment of replacing it and other upgrades would be.