When is the right time to buy Khans of Tarkir cards?

I am not going to repeat what I said a year ago, only tell you that the principles still apply.

Please reread my article from 2013: http://mtgolibrary.blogspot.se/2013/11/when-to-invest-in-fall-set-theros-for.html

Also notice how well the principles did apply to most Theros cards except for the ones that went into Standard decks that relied on RTR cards (such as Thassa, even though I think we might see her again).

Spoiler: The right time to invest for Standard purposes (rotation is almost always even better, but you don't want to wait until October 2016 to get your KTK goodies, do you?) is in December this year.

Khans cards will now - with a few exceptions - fall at a slow and steady pace until December.

Good luck!