$3.28 variable cost per ride is less than what Uber is paying its drivers (median pay is ~$10.88 per ride). I imagine it nets out significantly in SDC's favor after adding back in cost of gas, insurance, and maintenance. And, the $300k should decrease over time.
That includes electrical and maintenance. That's still less than a regular car, but it's still a larger number than most people consider.
It would be possible to say its less, though I'd tend to suggest it would be more than the Prius since there's no human custodian of the car to handle the "pull over, I've gotta puke" situations which can add unexpected expenses.
There's also things like fines when the car doesn't observe the laws (who gets the ticket if it causes a traffic jam, cuts off a bus, running over a fire hose, or fails to give way to a pedestrian in a crosswalk - https://www.forbes.com/sites/jimgorzelany/2018/03/29/first-a... https://sfist.com/2023/01/31/sf-regulators-have-had-it-with-... )