PM me - we just bought a 50 a few months back. Bottom line is you're going to have to spend $7.5 to $8.5 million to get a early serial number with a good history and original avionics add a couple million if it's got a Proline 21 package.
The market appears to be changing - aircraft with original avionics and aircraft with the Proline 21 package. Currently 10% to 15% of the DA-50 fleet has either been upgraded or is scheduled to be upgraded. So far, to the best of my knowledge, no DA-50 with the PL21 package has been sold but the best guess is that the mod will retain at least 80% of its value.
This probably goes without saying, but the absolute most important single thing that a buyer can do is get a good pre-purchase inspection. It will cost some $$$, but it will save you more than it costs.
LS