Sorry, I have no idea about $$.

Mine is intermittent, so it's about impossible to troubleshoot until its next episode. It usually malfunctions while I'm on the road, and behaves when I get back. :p

I generally suspect wiring connectors first, as they can corrode and lead to problems, and it's usually easy to unplug, look for corrosion, spray some contact cleaner and reconnect the plug. Sometimes just the act of unplugging and reconnecting 'wipes' corrosion off the pins and everything clears up, like magic, so it's my starting point.

Going by memory now, and I have a W126 body, your W124 may differ...

An rpm sensor is mounted on the bell housing, below the oil filter, driver-side, and senses flywheel teeth passing under it. If I remember, it is low voltage (2-3 volts) and a variable frequency corresponding with the rpm. A simple voltmeter test of the sensor confirms the sensor as GOOD/BAD.

The idle control actuator on the IP is also low voltage, 2-5 volts, depending on output from the Electronic Diesel System control module. Again, a voltmeter test. I think this test confirms EDS module output.

The EDS module sits behind the front passenger kick panel, right side, below the glove box.

I have not leanred exactly how and where the tachometer obtains its signal. The wiring diagrams show a connection with the EDS. Perhaps a voltmeter test could confirm voltage or lack of voltage to the tach.

There is a 'trim' resistor under the hood, behind the master cylinder, that is set to one of several positions to adjust the controled idle rpm. Uncontrolled, or mechanical low limit, is 500 rpm or so, controlled is 625-725 rpm or so.

There is a plug with two wires at the rear of the IP. Unplugging removes the EDS signal to the idle speed actuator, leaving the uncontrolled mechanical low idle.

As I understand, you can drive the car forever without the tach, and it will idle, although slower, without the EDS functioning.

Good luck
Jim