Mercedes S500 Ignition Control Module

My car was running fine, then stopped and would not turn on.
Mechanic replaced (in this order):  
Camshaft Sensor - No spark
Distributor Caps and rotors x 2 - Car started, ran for 45 minutes, turned off to
do oil change, and then would not start
Crankshaft Sensor - No spark
Said new Crankshaft Sensor that was replaced was bad - replaced with a
different new crankshaft - Car started, I drove home, started again later at
night and ran for 20 min.
Next day - Car did not start
Took back to Mechanic
He was stumped, and then had another mechanic look at it and was told it's:
Ignition Control Module -
Was told he has never seen one go bad.
New one, from dealer, is $4,000 +
He quoted me a used one (with one year warranty) for $750.00.
Questions:  Could this resolve the issue, or will it be something else after?
$750.00 too high?  I found some on eBay for almost half that price, but not
sure if I'm being overcharged by mechanic or .....????