I paid $750 for my last 226, NIB w/night sights. I've seen them as high as $800. I think the last NIB 239 I saw was around $750, and it too had night sights. I bought my 239 used, in 99% shape, w/ night sights for $500.
If your going to get rid of the 239, I'd suggest you try and sell it outright, or on consignment before you trade.(assuming a dealer trade) You will almost always do better selling it than trading it. Dealers usually give you about half of what you paid or its worth. They want to realize a profit too, and usually put it back up for sale at what your hoping to get for it.