If your '80 300SD is anything like my '82, I found that the MB dealer charges $100 for the bracket.
Part number(s) 617 090 0140 for W126, and 617 090 0340a for W116. I just priced this item from Parts Shop and the quote was $69.
However, there is a guy on eBay who has been selling these for about $34.95, and $8 for the grommets. http://cgi.ebay.com/ebaymotors/aw-cgi/ebayISAPI.dll?ViewItem&item=541531270&r=0&t=0
Or you can get the same item from Euromeister for the same price, $34.94, and an additional $9 for the grommets. You can call them at : 1-800-581-3327. They are in Castro Valley, CA.
But, before you replace this bracket, make sure your motor mounts are good, and that they are tightened down, or you will have wasted money on something that will break again.
My information is provided for comparison purposes only, and not posted solely to undermine the sales department at Parts Shop. I would like to suggest that we all buy our parts from Parts Shop whenever possible, as they host these forums for free. So, you might email Parts Shop and see if they can price-match on this item.