All Parts Express (under buy parts above) has the Bosch set for $102, includes the coil wire. I've found Bosch to be very accurate with replacements, they are an OEM supplier to Stuttgart, after all.

Dealer parts markups are outrageous, most of them around here charge more than suggested list.

Here's the link at APE: http://www.allpartsexpress.com/ShopByVehicle.epc?q=1990-MERCEDES--BENZ-300se--/--L6_3.0l-Ignition&yearid=1990%40%401990&makeid=MERCEDES+BENZ%40%40MERCEDES+BENZ%40%40X&engineid=1194117%40%40300SE++%2F+L6_3.0L%40%40300SE&catid=Ignition&subcatid=Ignition%20Wire%20Set&mode=PA