You know that any insulation improvements at this point would pay for themselves inside of 5 years. Better windows, insulation under the floor, inside exterior walls and in the attic or along the roof on the inside... even better doors or adding storm doors all go to help. There are ways to make this inexpensive and unobtrusive.
It would save you a fortune every month. At least $300/Month. That's at least $1,200/yr for the winter months. The insulation itself isn't that expensive either if you install it yourself.
The most expensive part is the windows. Those are extremely expensive but necessary. Double and triple pane insulative windows aren't cheap. At least $1,200/ea. Buying "storm windows" to go over the outside of them is much much cheaper but doesn't look as nice.