The Center Muffler you are referring to, costs from $100 - $200 OEM. Yours should be in the upper-half of this range.
I highly recommend to get the OE parts. They last much longer and are more cost efficient.
It is always a good idea to replce the entire system if parts are starting to rust out. As soon as one goes, the rest are sure to follow. Also, it is very difficult to weld in a new piece when the pipes are rusting up.... the welds will not hold and the new part runs the possibility of falling off.
76 240D (W115) - 555K miles
78 300D (W123) - 200K+ miles -- SOLD
[This message has been edited by WmHarlow (edited 11-26-2000).]