What's the fair cost for ABS sensor replacement?

One ABS sensor on my '88 300 CE had gone bad. Mercedes dealer did estimate and said $435.00!!! Where sensor is only $150. Why do they charge nearly $300 for labor? It does not look like it's 2 hours job! Parts shop sells sensors for around $120, but how big of a deal is to installe it? Where is the exect location of the sensor> How do I check wich one is bad? Fortunatelly the winter in Chicago is not so bad, but little oulsation in the brake pedal drives me crazy! I have to fix it a.s.p.

Thank's for any advise

Daniel