Really no way to tell until the work begins! The line is going to be rusty, the point it attaches to may need replacing as well then the next point or connection and so on. The rear bleeder screws may not turn so the system can be bled, that means new wheel cylinders. As you can see it is no way to give an estimate. Clock time plus parts is how this is usually done. If you are real lucky and just that one line can be replaced and the system can be bled, cost goes way down, so does the quaility of the repair. But still can't guess at a price.
Once i overhauled both wheel cylinders on an old chevy, went to bleed the brakes and guess what! That was way back when i just started doing this. That was a good lesson, always check the bleeder screws first thing if you are going to need them to turn! Course now dont overhaul anything, just replace it. You remember them days when we fixed all that stuff. Rarely replaced any parts, these kids today have no idea!!
Dont know how i got down here, must have pushed the wrong button, lol. Only parts store around out here in the country was NAPA. I miss those days,,,before the rat race! Man what a difference it is now, cant say it's for the better either. Sorry, got to talking to myself i guess!