I just had the top end done in my 87 300SDL. Like you, my car started mysteriously loosing coolant. Had no visible leaks, didn't smoke, etc.
These prices are based on memory only, but I've posted extensively about this. Just do a search on my name.
I took the car to the MB dealer in the town my office is located in. At first, I was told it needed just the gasket and estimated the job at around $1800. After it was taken apart, the head was found to be cracked in a few places. The MB part was well over $2000 for a rebuilt head.
I ordered a NEW head complete with valves, guides, springs, etc for around $2200 delivered from MetricMotors (Www.metricmotors.com
I went ahead and had them do the head, gaskets, and all new valves. By the time I picked it up, the total was around $5200.
Sold: 1987 MB 300SDL, 320k miles, Astral Silver, Blue Leather, all books and records from day one.
Totalled: 1998 VW Jetta TDI, 488k miles, Classic Green/Gray Leather, loaded, E-Codes, "Slightly" modded, "Slightly" faster than stock, arrived 11/07/1997.
MBCA Member, Chicago Region