They are expensive because they are hard to get. Newegg does not seem to have them. Amazon only have used as far as I see. They are probably not produced anymore, so availability is scarce.
CPU Support pages: (both need BIOS version 7.10 for Kaby Lake)
http://www.asrock.com/MB/Intel/Fatal1ty%20Z170%20Gaming%20K4D3/#CPU
http://www.asrock.com/mb/Intel/Z170%20Pro4D3/?cat=CPU
As always, it depends on the given CPU how high you can clock it.