Well, I think we've put on about 83,000 electric miles since then. Note that we had the most efficient gas cars possible (Honda Insight and Prius PHEV conversion) so gas savings aren't nearly as dramatic as they could be - but MAN have we had a lot more fun driving. And we've been able to carry so much more in the Model S. Even the Roadster is a better road trip car than the Insight.
We have probably saved about 1,383 gallons, or say roughly $4,850. Actually probably more than that, as I doubt my wife would have stuck with the Prius this long.
If we had driven comparable gas vehicles instead, it would have been more like 4,813 gallons, close to $17k.