I would guess its worth $37-38k...
I've been following these for a bit and it appears they have had their initial big depreciation drop with the intro of the new 3 series... kind of sad, but I don't think there is enough of an enthusiast market to keep the used diesel prices where they should be.
If you find the right person you might get $40k
Herbert - '12 X5 35d / Sapphire Black / Black Perf Nevada / Aluminum trim / ZPP / ZPS / ZCW / ZAV / Apps / 3rd row
Betty - '04 CTS-V / Black Raven / Black Leather & Suede / 6 speed LS6 combo 400 hp & 400 tq
Sterling - '06 Silverado HD / White / Grey Leather / LBZ Duramax - Allison 6 speed combo / Crew Cab Short Bed LT3