If you like, you can skip directly to where you can buy this part.
We sell good, original BMW, used E30 3-Series 325e 1984-1987 muffler and rear exhaust units. The price is $90 plus shipping.
Pelican Parts, a vendor whom we like and from whom we buy personally, offers an Ansa-brand muffler new for just over $230. As I recall, this brand is a decent-or-better-quality muffler.
GK Euro Parts offers the same part for just under $270, made by … to quote them: “Eberspacher, which is the Original Equipment Supplier for BMW. It is the exact same thing you would get from your local BMW Dealer, but at a much lower price. List price for this muffler is $446.95!!! Part Number: 18 30 1 707 046.”
We could find a cheaper new unit on eBay too, for just less than $170, but I didn’t conclude from the listing that this was for a high-quality part.
These are all reasonable alternatives, I think.
Another option is to go remove the part from a help-yourself junkyard, but the amount of work involved and the skill level required, are not trivial.
It’s probably safe to say that our used, original BMW units are price-competitive.