Using Texas as an example:
10-80% on a Model S/X is 70kwh. Rate billed to customers will vary from 30-50c/kwh at time of charge. Spot in Texas typically ranges from 5c to 15c/kwh. Assuming charge equipment is fully depreciated all they need to pay is maintenance and network cost, which is marginal across a bank of 20 chargers.
70kwh x 25c = $17.50.