The answer is: They seem pretty everyday rubber to me. I went specifically to my local dealer and was charged $2.50 EACH for this roughly 1" round o-rings (looked to be the same as in the package of 10 for $1.99 at the local parts store).
What fooled me was the original ones were squared in cross-section and hard. Well they were squared from compression and hard from being cooked by coolant over 10+ years.
Maybe the Benz ones are may out of unobtanium or something ....
Live and learn,
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)