Help identify a part on the underside of car

I'm trying to find this part name/number to get an idea of the price because the mechanic I took it to quoted ~$200 for the part and $40-50 for labor, which sounds ridiculous considering it doesn't seem like a large part or difficult to replace, but I could be wrong. The part lies on the underside of the car directly below the driver side door and passenger door and is placed between the front wheel and back wheel. Half of the part broke off as I discovered the issue while driving on a highway so a fastener must have come loose and half of it snapped off while I was driving. The mechanic told me the part is used to block water from entering the underside of the body. In the picture, I secured the part that was left. Is it attached to some other part? I was told the other parts on the underside were fine so I don't understand why it would be so expensive if it's just a plastic strip.

Thanks for your input!