As most have said, every shop is different.
I recently got a custom build done. I did the basic level of haggling and got 10% off the frame, 20% off parts and a free bikefit and saddlebag. All for just asking “what can you do for me?” Have to say it was great service from start to finish, and I will be recommending them to anybody that listens.
I tried pricing it up to build myself and couldn’t do it much cheaper 🙂
On the other hand, I would haggle over low cost items like clothing etc.