Wow this is awesome!!! I think the base cost should be based on the Death Cult Assassins. They are the lest amount of points, and if someone wants a less expensive assassin, then they could have it.
For more inspiration, there is a great article on Wikipedia (I know, not the most reliable, but still) here that tells a bit about the 4 well known assassin temples, and more...
http://en.wikipedia.org/wiki/Officio_Assassinorum
In particular: "There are two more major temples that do not have miniatures in the game, and thus not much information about them has been compiled into official sources. One of them,
Venenum, deals in assassination through poisoning. Of the other,
Vanus, nothing is known (the Latin word "Vanus" means "Empty"), though many fans believe they deal in assassination through sabotage.
The Officio Assassinorum also maintains patronage over numerous minor assassin temples and death cults."
So, let's get crackin' 40k Terra!
__________________
Lord of Fluff and Blood Angels
Herald of Nurgle
"I wield my power with the Emperor's Authority. Those who would say that I am 'radical' merely have minds too small and impotent to realize all the weapons at their disposal. Do not question my methods on account of these so-called 'Puritans'."
-Inquisitor Mathias Rosenadel