Someone can add this to the Wiki.
Flying out on TK6 ORD-IST (08/2015). The door to the staff/back room was wide open by the Business/*G counters, on the whiteboard:
"All upgrades: $999, except X, N"
So I assume that's US$999 to go from Y->J (as Comfort class isn't offered anymore on this route), all booking classes apart from X, N (which I assume are award redemption/staff travel classes)
Didn't even bother asking, personally think that's a tad much to be a good deal.