When I bought mine in 2010 I was originally looking for a CPO diesel, but the prices were so close that the tax benefits you mention made the new car cheaper. Fast forward to today, I've been pricing VW TDIs and the difference between a new and CPO is only about $1500. So I'd say the diesel does carry a premium on its own, and also holds up as a CPO.
The thing we don't really know yet is how that translates to a high roller. There aren't many 100,000 mile BMW diesels out there yet, but if you want one I might know a guy who's pricing VW TDIs and might cut a deal!
