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This is no doubt quite a task, requiring many pre-analysis of hydrogeological and geological nature. Do you have enough water yield for a sustainable installation? Never the less as for cost you can get some guidance from http://www.engineering.lancs.ac.uk/lureg/nwhrm/engineering/turbine_costs.php
We have a few similar power stations in Sweden with very large heads, tunneling water from a lake, and I was (long time ago) involved in one of them in Jämtland (near a place called Frankrike).
In order to help you we would need to see your Feasibility Study, and overall design.