The operating cost of hydroelectric plants is relatively small. In the total cost, however, the cost of the construction should be included.
This includes the cost of building the dam as well as buying the land occupied by the lake to be created.
Many of the large dams serve multiple purposes in addition to producing electricity e.g. control the water flow and use.
The water after rotating the turbines is used for watering or urban water supply.
If the dam is carefully constructed and the water that leaves the generator is used (sold) for other purposes, the cost of electricity produced is very low.