What I don't really uderstand is how little the 1965-72 W109s are worth. Thanks largely to the demands of chief engineer Rudolph Ulenhaut, these cars were built to a standard of engineering and workmanship that had no peer in the day save the very limited production 600 Grosser. Even the vaunted 6.3 is at best worth $35-45k, and that's only if it can be proven that everything that needs to be done to put it back in tip top shape has been done.
And I suppose I have suggested the answer to my own question. It costs a lot of money to make these cars right, and when you are done you will definately be financially underwater.
But (except for the decidely underpowered 2.8 version), these cars are still an absolute joy to drive. The air suspension that is the primary difference between the W108 and W109 has been unfairly maligned. When the valves have been properly rebuilt, the car will stay up indefinately and the ride is still iremarkable. Once in good order, those valves and the air bags will last for many years; the only "maintenance" is to keep the alcohol reservoir filled and release accumulated mosture from the holding tank under the left front fender twice a year.
I too am underwater with my 74,000 mile 1972 300SEL 4.5 that I've owned for 10 years now. I'm entirely satisfied with the experience.

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Berfinroy in CT
Present vehicles:
1973 300 SEL 4.5
1959 Rolls Royce Silver Cloud I
1973 Citroen DS23 EI five speed
Past vehicles;
1959 Thunderbird convertible/430
1958 Bentley S 1
1976 ex-Max Hoffman 6.9
1970 300SEL 2.8
1958 Jaguar MK IX
1961 Jaguar MK IX
1963 Jaguar E-type factory special roadster
1948 Plymouth woody
1955 Morgan plus 4
1966 Shelby GT350H Mustang