You set the price of the job based on the materials and design of the job they selected. If they want the price to be lower they have to use less expensive materials or change the job specifications.
What does your customer know about pricing jobs like that? If they knew how to do it they would not have contacted you to do it.
You calculate how much time, materials, equipment, and subcontractors it will take to complete the job. Add your markup and profit on top of that and that is the price.
If the customer tells you have to lower your price

to get the job ask them what part of the job they would like to cut. Let them get a cheaper price from the next guy and live with the consequences, good or bad.
End of story.
Take a drive down to the gas station and tell them thier price is too high. See if they lower thier price because they have to sell you gas.