Jay, either you or your German guy are confused. The sensors that he is replacing for 6 bucks are the wear indicating sensors. They have nothing to do with the ABS sensors which vary but all cost over 100 bucks a piece.
There is one at each front wheel and one on the differential for 3 channel systems and one at each wheel for the four channel systems with ASR.
Bosch Master, ASE Master, L1
33 years MB technician