My seat of the pants market estimate
W210s 2k-5k depending on age, mileage, if it has a turbo or not. Personal data point, my 98 E300 is sale pending at $3k with 253k miles and just did the glow plugs last winter.

W211 05-06 (OM648 “CDI”) are now anywhere from 4-8k from what I was seeing while searching for a car a couple months ago

W211 07-09 (OM642 “Bluetec”) are now $5-10k. I bought an 08 E320 last year private sale for $9k

And I just bought a 2013 E350 BT from a MB dealer CPO for $19k so there’s another data point. This is the last year with the OM642. Non CPOs of this MY or especially the 2011-2012s with more mileage (thus disqualifying them from CPO) can be had in the mid teens.

The 2014-2016 E250s with the OM651 are commanding mid-upper 20s into the low 30s from MB dealers selling as CPOs.

The W212 diesels don’t stay in stock long at the dealers. I literally had to call and get my name in as soon as I got the email alert from the MB CPO site. I lost out on a few before jumping on it radio station contest style.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech