Insulating the cage will do a lot. Take all the advice people give for retaining heat in glass tanks, and apply it to your PVC cage - foam insulation board, etc. And radiant heat panels are great for efficiently and safely bumping up the ambient temperature of a cage in a cold room. I'd actually go so far as to say maybe you should think about using the RHP for the *cold* side and a basking lamp for hot side during the day. In the UK it's much more common to put a lamp inside the cage, with a wire mesh cage around it so the snake can't reach it and get burned. Doing that will keep more of the heat inside the cage than putting the lamp above a screen-covered hole in the top.

Even if you spend more than you'd like on getting that stuff set up initially, it will be way cheaper than running a space heater all the time in the long run.