It’ll be less than an hours shop time. Most shops around here charge a minimum 1 hour, so I’m guessing it’ll be $100-150. The line itself should be less than $25.
The most important thing about this is the other lines will be checked. Brake lines, like all things elastomeric, get old and fail. One leak on a vehicle this suggests that the other lines might be getting old too. All four lines could be replaced and the entire system bled in one hour’s shop time.