A couple months ago, I picked up a whole stereo which included KG4 for $200. While i was picking it up (was listed on CL) the owner told me he had calls from two guys that wanted it and one said he'd pay $200 for just the KG4.. Hmmm, I asked him for the phone numbers of those guys, which he provided.. As I pulled out of the sellers driveway, I called both and offered them for $100, if picked up right away. I explained the situation and said I didn't buy the system for the speakers..
Anyway, I met the one guy on the way home and sold them for $100.. I knew that was a little on the low side, but I needed another pair of speakers like I need another 14" of snow...
In reality, I think you're right on.. $100 is low, $200 is a little high, but they'd probably sell sooner or later for $200. $150 is a decent price, if they are in good shape.