Good question! For an equal consumption, the intensity of the current does not affect the cost of electricity. However, since twice the electricity can be consumed with a 400 A distribution panel compared with a 200 A panel, we install a meter designed to also measure power demand.
If the demand exceeds 50 kW during a given period, the excess is billed based on the terms of the rate in effect. We then need to issue the bill on a monthly basis to correctly assess the differences in power demand from one month to another.
This is not required for 200 A panels because the maximum power demand is 50 kW.