I'm having an empty upstairs room converted to a bathroom.

The contract has a plumbing line item, $3900. Separate from fixtures.
The contract specifies all copper for the supply lines.
Without telling me, they installed the plastic stuff instead.

I realize it's hard to talk specific pricing without seeing the job, but:

How much should they knock off for not using copper? They said on the
phone "you'll get a credit, there's a big difference in the material and
labor."

There are six fixtures to feed with the supply lines - a five drop
bathroom. There was no plumbing upstairs before this. It's a big
bathroom, about 9x18.

If anyone could give me a percentage difference for installing one
versus the other, even that would help. I'm just looking for
information to give me an informed view of this.