I know BavAuto sells them for $240, I cant imagine them being any more expensive anywhere else. I'm curious as to how hard it would be to remove the old valence and fit the new one. Mine is pretty rough and if the rest of the car comes together it would look much better.
This is a link to an aftermarket part:http://www.innerauto.com/store/buy.php?make_text=bmw&ml=&model_text=325_series&year=1991&part_name=lower_valance&brand_name=replacement&repcat=true&s=1The part number is the same in realoem for the cabriolet and the is coupe. It's $190 but I've seen them for even less on the web. I couldn't find the main supplier but I think they want $160. I'd check this first from the supplier to see if they can give you the BMW part number to verify.
If you get an aftermarket one, make sure it is at least capa certified or else you might have fitment problems followed by rust issues in the long term.