I just bought a(nother) P239 9mm on Gunbroker for $600. It's used (probably an LE trade) with some minor holster wear, no special finishes or bells/whistles. One mag, no box.
As someone who's often on the hunt for P239s, that feels like more or less the "going rate." If you get a worn-out POS, it'll be less. If you get one that's more pristine, with a box, etc.... it'll be more.
This brings my P239 total to 3.
I've got one in .357 that I've been carrying and shooting the last few months. Now, when I switch back to 9mm, I'll have two to work with. Figure they ain't gonna get any cheaper.