Since Tesla already rations kW between chargers at many sites based on demand by lowering kW deliveries when more BEVs are plugged in, a time simple per-minute-fee might be a viable option.
Tesla could adjust the fee depending on the cost of electricity in the area.
For example, in high-cost California, at a site with six chargers, 360 kW total capacity, and 120 kW capacity per charger, it could set the fee at 50 cents per minute.
This would result in a variable cost per kWh of 25 to 50 cents per kWh, depending on how many BEVs are charging, which would might at least come close to TSLA's actual delivery costs per kWh.
This would also incentivize drivers to avoid charge sites already near full occupancy, and also to leave earlier as the last empty chargers filled up.
In related news, Musk hinted at model 3 DC charge fees last week:
Elon Musk says Tesla Model 3 will include ‘free long-distance charging’
...the Model 3 will not qualify for lifetime free supercharging, however in answering this question Mr. Musk stated:
Model 3, from the beginning we said free charging is not included in the Model 3 – free unlimited charging is not included, so, free long distance is, but not free local. It becomes really unwieldy for people to use the gas station approach for electric cars, like, cars should really be charged where you charge your phone, but then you just need to solve the long distance problem which is what the supercharger stations will do.
We suspect this means that they will be on a similar program to the Model S, with a certain kWh allowance per year – only perhaps with a lower limit as the Model 3 is expected to be a more efficient vehicle. Tesla has responded that they have nothing further to announce beyond what Elon said on stage.
You can see a video of the meeting with Q&A here. This answer starts about 15:30 into the video...
https://electrek.co/2016/11/17/elon-mus ... -charging/
TSLA has also begun to try to get payments from site hosts:
Tesla is increasingly able to make businesses pay for the electricity delivered by its Superchargers
https://electrek.co/2016/11/16/tesla-su ... ectricity/
This may soon be a problem as well as an opportunity for TSLA, IMO.
Once site hosts begin to actually have to pay to charge Tesla BEVs, they may soon to realize that they can optimize the costs and benefits to their businesses by offering DC charging to all BEV drivers instead.