It gets cold in New England and our house is kind of drafty. We got a couple of heaters similar to these and sprinkled them through our house:
http://www.lowes.com/pd/Lasko-Black-Ceramic-Tower-Electric-Space-Heater-with-Thermostat/999919474
They probably are a little less expensive than when we run the AC in the summer as you don't leave them on all the time. The room gets dry if you do.
Interesting data point - thanks for the info. Good to know that your bill doesn't rise above that when using AC in summer using those, but at the same time it sounds like you use those as an ancillary heat source.