I would like to install a wash room in our attic.. the plumbing runs through the attic... how much would this cost?

There is no way to give you even a rough idea. You could be in NYC or Alabama. Union or non union. Are you talking about a vent stack in the attic, or water and drains? If it is a main stack it depends on whether your locale allows wet venting and running a toilet down a main stack, at least from an attic, is not a good idea because that much water flowing down could blow water from drain traps.