In the Charging Estimator section on their website, they think that a Model competitor gets 21 mpg (https://www.tesla.com/models) and gas is $2.70/gallon, so I would expect it to be based on that.
True, but if the cost of gas jumps back to >$4 where it was 2 years ago, will the cost of a supercharge go up as well?
If that SC unit is powered by a natural gas power plant, I would assume so.