I have thought about this previously...
I don't think the $.07 rate can be subsidised significantly from the purchase price, except perhaps for early adopters...
When we look at a solar farm LCOE is $50 MWh or $.05, so with a solar farm Megacharger Tesla can potentially breakeven.
Selling solar and services to the grid, and Supercharging would also be profitable,
A build like that is capital intensive, even with Megachargers spaced every 250 Miles.
Analysts at Bloomberg New Energy Finance say the lowest-cost projects financed in Australia, China, Chile and the UAE in the last six months hit a levelized cost of energy of just $23-29/MWh and the best solar and wind projects will produce electricity for less than $20/MWh by 2030.
www.pv-magazine.com
(This is April last year, the LCOE is probably lower now.)
The other alternative is Tesla may be able to enter into long term fix price supply contracts with solar and wind farms...
Every Megacharger will probably have at least 1 Megapack, maybe they can make money or save money with just the big battery and a smaller solar installation.
I'm aware solar isn't going to work in every location.
Megacharging itself is probably 4x250 kW not that different from charging 4 cars at the same time, the economic side is more challenging.
The other reason I favor solar farms with big batteries is the ability to "Island" and remain operational if the grid goes down.
My other thought is trucks may schedule charges in advance, the electricity for the charge needs to be in a battery hence guaranteed and trucks don't want to wait around... So not only is their an economic challenge, there is a service level challenge.