As the previous post indicates, I am now measuring the kWh used to charge the car. I've been keeping track of per-day costs, and here is the chart for the first couple of weeks. As you can see, the cost per mile does not correlate well with the miles driven - which means that the theory I had about the rebalancing contributing a major portion of the cost for short trips seems to be incorrect (at least, for the length of trip that I've been taking).
The gap in the chart is for when I watered the batteries - no miles on that charge, so no wH / mi.
Over this period, I have driven 111.4 miles, and used 54.9 kWh to recharge. This nets out to an average of 493 Wh / mi (as shown by the green line on the chart). My electricity costs 7.5 cents per kWh, which means my average cost per mile is 3.7 cents, which, at $4.00 per gallon, is the equivalent of 108 MPG.