I think they're about £40 brand new from the main dealer. This comes with the dipstick, cap and internal filter. Reasonable price IMO. I'm not sure if the 500E and the standard W124 ones are the same though?
Spring has to be purchased separately but is relatively cheap.
Thanks for the info, but thats the wrong shape. My dealer quoted me £110 for the reservior and the cap/ dipstick. The one in mine is not cylindrical, more square in section, and slanted forwards.
Thanks for offering to trawl the garage: A fellow member breaking a w124 actually saved you the trawl: but thanks for being so quick off the mark. It' a rare thing when you can put a call out and pretty much immediatley get a response.