I think you mean it will _gross_ Tesla $20. From my research, Tesla is (currently) not making much on energy delivered. That _will_ certainly change in the future though.
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.
Since the Tesla-Supercharging business is running at break-even, you're looking at something like $8.40 for a non-Tesla adding 70kwh.