I replaced one a few weeks ago (it was starting to make noise), I think it was a couple $100 (I had bought it on-line a while ago). In my car it made a difference at idle. When mine didn't work previously (due to a CCU issue), I could feel the difference in the temperature when I stopped at a red light. It didn't get cold, but I could feel the change and it was annoying.

Edit: Here it is for $164, it's a little cheaper than I remembered:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ42BK0IHE4W&year=1985&make=MB&model=300-DT-001&category=All&part=Auxiliary+Water+Pump