Can't powdercoat plastic. And rattle-canning such a small surface area in matte black after proper surface prep is really not difficult to do, even for a total amateur. As long as the final coat is done slowly and with care making sure the passes are overlapped well, there would be no discernible difference (including durability/longevity) between your $30 job, a $200 job by a professional, or a $500 OEM part replacement.
I don't see where you are getting this $500 oem part cost? The rear valance is between 127 and 139 and the front is 159 to 180.