I think with elite advances I would consider the character, and if they had used the skill involved. For example, I would make it cheaper to aquire basic weapons bolt, if the character had used on already on a mission, than I would if he decided to just buy it. That said, anyone wanting to finance a bolt weapon is welcome to buy the advance!
Psychics work somewhat differently. Psychic supremacy is a very powerful advance. I think a fate point and experience is not that high a price to pay if they really wanted the advance. Should a player only have one fate point left, I might have to do it slightly differently, as that would be unfair to ask them to sacrifice their only point.
While on this topic, what would peoples opinions on allowing the purchase of psy rating as an elite advance for other careers be? Provided they had reason to do so in the story, not just buy out right with no reason. [Im thinking eldar farseer unlocking psychic potential here, along those lines]. I only ask because nascent psycher is rather decent for no cost, provided you only use when its safe to do so.