The cost to repair the brake fluid leak will vary depending on where the brake fluid is actually leaking out of.
If your master cylinder is leaking then it could cost $100.00 to $200.00 while a simple brake line may only cost $80.00 to $150.00
For a complete brake repair job can cost from $500.00 to $800.00 for the parts and labor.
We'd need to know more information on where the brake fluid is leaking to give a better answer as it really does depend on where the brake fluid is leaking from.
But expect to pay a shop from $100.00 to $500.00 minimum to repair the brake fluid leak depending on where the brake fluid is leaking.