Pictures or links to the part would help and a description of what you're talking about. I found the part and a picture of it on ECS Tuning.
I've replaced both the left and right pieces on my Z4 M Roadster but this was a few years ago. I remember that one side was around $50 but the other side, nearly the mirror opposite, was $100. Apparently the price has gone up.
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The Coupe: 2016 M4 | Sakhir Orange | Black Full Merino Leather | CF Trim | M-DCT | More | ED 5/13/16
The Sedan: 2018 M3 | San Marino Blue | Black Full Merino Leather | CF Trim | M-DCT | ZCP | ED 7/18/18
The Roadster: 2006 Z4 | Interlagos Blue | Black Extended Nappa Leather | Carbon Leather Trim | Purchased 7/19/12
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