Hello Mary, welcome to the site.

Exsulating seems to be the easiest method to drastically reducing energy consumption. If I had to guess it might also be the cheapest. I haven't compared. It basically involves taking off all of your siding and adding about 4" of foamboard insulation to the exterior of the house, and then residing it. This not only adds R20 to R26 without any thermal bridging, but it also ensures the house is sealed up pretty well. Thermal bridging and air sealing are both major factors in passive house design.