WannaDiesel I have been looking at Mercedes prices for about a year now. All I can say about the books is that they are useless. The prices for these cars are all over the place, but in general here is what I have observed, for New England at least.
Good 300SDL's are rare and go for 8-9k. For that you get a sub 150k car that is mint.
1982 -1985 300SD's in mint condition go for around 7k. Real nice ones go for 4-6k. Good drivers can be found for 3k or lower.
I'm not to sure about the w123's as the w126s are the ones that I am intersted in. They seem to follow the 300SD's maybe a little cheaper. I have had good luck searching autotrader.com and ebay. I hope this helps.