How much should repair for leak in oil water separator in 09 Mini S cost?

Not sure of the term "oil water separator" on this vehicle; for a concern such as lack of power or misfire up hill, would recommend a test be performed to find out what is going on. You could consult local shops for a quote also -- https://repairpal.com/repair-shops/mini-repair-and-service-in-nashville-tennessee