Try to visually compare your tank to alternatives. When I needed to replace the overpriced tank in my 190D turbo, I found that there were other Mercedes cars with nearly identical tanks. I eventually discovered that the 500E tank, despite having a different part number, was identical in every respect except the corners were a little more rounded and the price was half. Mercedes seems to invent the same tank over and over again, so a little persistence will help.

http://www.peachparts.com/shopforum/diesel-discussion/369176-anyone-have-spare-190dt-expansion-tank.html