Bring the strict preview of the GM as to both the cost and if the player, is infact, allowed to buy them. How do you charge or would you charge for them?
Example.
How much would you charge for an Imperial Psykers 2oth power, his 10th in his second discipline?
Or
Psychic Supremacy for the same character.
Now, how about just a simple extra power? and Psychic Supremacy for that character?