My tech charged me $140 about 2 years ago for the flex disk parts and labor (as well as a faulty brake sensor).
That is pretty good.
I took my car to the mechanic this morning and he is replacing the front flex disk for me in two weeks (busy place and he has to order the part).
As I cannot take a new disk to him (like taking a piece of meat to a restaurant and ask them to cook for me), he ordered form the local MB dealer 60 miles away. For my 91 300E, the disk lists for $95 and with shipping, it will costs me $107 with shipping and his mark-up.
If I buy it myself, I think the best I can do is around $80.
So your price of $140 with labor is excellent as I can see there is easily 1 - 2 hours of labor involved in replacing it.