You've always been pretty wise for a sweatyboatman so riddle me this... if Kraxis can never cast this spell in a standard game, why is it in the game? If it was only meant to be achievable by custom factions, then why not just let it be from some mod instead of the standard game?
I don't know about wise 
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Just to clarify, by "less than once per game" I meant on average. If 500 puts it completely out of the range of possibility, as you say, then it is too high. I've never really tried to generate influence, so I am not sure of the mechanics involved. Like I said, I would put the cost of that spell so that it requires good resources and gets you one and if you're really lucky two shots per game.
If you drop it down to a more manageable 100-200 range, a good random map might end up giving you 4-5 opportunities to use that spell.
Can you imagine the outcry on the forums if the AI started using that spell? Oy!
IMHO, influence in the game suffers from not having enough use in the game. What else do you need it for? You often can't even trade it with the AI. If there were other productive uses then I would feel more comfortable dropping the cost. Because there'd be a tradeoff to using it. If you use Influence Spell A you're giving up the opportunity to do X, Y, and Z. Right now there is Spell A.
