Well, I guess the title is somewhat misleading, but here it is.
A Nightbringer is like 360, and a deciever is like 300 (I don't have the codex in front of me, so these numbers are approximation) But by how many points a game should be in order to have a C'tan EFFICIENTLY put in? Like I'm thinking about a 2000 point game then it'll be good to put a nightbringer in, since your c'tan is not worth 1/4 of your army!
Another thing is, I'd rather put a Monolith first before I put a C'Tan in, since monolith is more everything killers, whereas C'Tan is more bigtime HQ vs. bigtime HQ type playing. (ie. Abbadon vs. Nightbringer)
What do you think guys?