You are right -- my arithmetic is wrong.
We can probably agree though that breakeven is at $5,000/$0.28 kWh = 17,857 kWh, or over 10 years ~ 150 kWh a month
That sounds about right. 3 supercharger stops a month. That's about where we're at I'd say! Some months are a lot more but some are a lot less. I'm going to see if I can check on Tesla's website to get a printout of all my supercharger stops just to see if that rough guess is accurate.