"since I'm home all day, it would start costing us a pretty penny to keep the house at comfy temperatures, instead of the usual no one's-home level."

Got any spare carboys lying around? Fill them full of water and put them somewhere where they will be warmed. This will increase the thermal constant of your house, and it will take longer to cool down (and longer to heat up).

Consider making some interior storm windows, especially if you windows are leaky. Here is my design: http://www.midcoastgreencollaborative.org/Documents/storm_pamphlet.pdf

Seal air leaks (like outlets and the pipes from the basement).