The first thing I did was pull it apart. At $24 it's the cheapest diplexer I have found, so I figure it can't be too hard to DIY.
Inside you have what you would expect to find, a series of inductor coils and caps. The VHF side uses a 100nF cap the UHF side 47nF. Unfortunately my Fluke doesn't do mH so I'll have to measure the inductors some other time.