My aftermarket Y-Pipe I installed did not fit well going in, taking a crowbar to align it. Needless to say after in it leaked. I'd like to see why he is selling the Y-Pipe to start with. I then had an OEM pipe installed and it worked fine.
I had more work than just the Y-Pipe done so I don't know the portion of the bill that went for that, but I think about $700 total from a shop may be a fair price. I'm guessing the $250 for the pipe and 4 hours of labor.
There's other stuff like getting the turbo cleaned since its out, maybe another hour of labor. Also, taking a look at the exhaust manifold and see if it is leaking with black smoke stains where it attaches to the engine, which would mean a new seal and or manifold is needed.
If you do decide to do the work yourself, moon wrenches and a super long extension bar from underneath is how I loosened and tightened the bolts.