Without seeing the job, it would only be a wild guess.
You may want to get three looks or bids on that.
I have roughed in drains in my garage slab already and I am looking to hire a contractor to complete the rough in now that I have walls framed. The work would be:
- connect vents for lav, toilet, and shower to main stack in the attic.
- run supply lines to fixture locations and stub out.
Note that the new bathroom is directly below an existing bathroom, so the vent stack would only need to run about 15' straight up before being connected to the house stack. The supply lines for the upstairs bathroom run right by the new bathroom, so there will be short runs for the supply lines.
This work is being quoted at $3800. Does that sound reasonable? From my reading, it looked like bathroom rough ins were running $3-5k, but this one seems pretty easy since the drains are in place and the supply and vent hookups are not very long. BTW, this work would be in the city of San Francisco, so that would add to the cost.
I have posted a few photos of the jobsite. The first is the existing drain rough in. The second shows the proximity of the new bathroom to the existing DWV stack and supply lines. Hope this helps narrow things down.
One thing I would not do is give you any guarantee that it will work afterwards since I would not know how you did the piping under the floor. I cannot see what you have called the "house vent" but unless it was installed with the intention of it being for a downstairs bathroom, it will not be a proper point to make the connection. IF it is going to be cast iron, then it will be considerably more than a PVC/ABS system. You are being logical when you assume that since part of the work is done, the remainder should equal an original cost less that portion. Have you ever tried to price what a Cadillac would cost if you assembled it by buying the parts. The sum of the pieces ALWAYS is more than the price of the whole, whether it is a Cadillac or the plumbing for a bathroom.