I have seen three different discussions of Custom Powers/Proficiencies in custom race/class creation.
This post about creating custom races essentially says that a power/proficiency - which are one and the same - costs 40XP:
The Player's Companion p.79 says:
"FIGHTING VALUE TRADE-OFF EXPERIENCE POINT PENALTY A class with a Fighting Value of 2 or greater that makes a tradeoff increases the experience point cost of its Fighting Value by 150XP per custom power gained. This is because once a class has a Fighting Value of 2 or higher, it has so many weapon, armor, and fighting style choices available to it that it can specialize without suffering much loss of effectiveness."
So the cost here is 150XP per power + a loss of some fighter abilities which are traded off for it.
The Player's Companion pp.79-80 allows a direct tradeoff, with no penalty, between Thief Skills and powers, on a one-to-one basis. 3 skills are 200XP; 5 skills are 400XP; 10 skills are 700XP; and 15 skills are 1,100XP, for an average of 72.25XP per Skill/Power.
I am trying to figure out the rationale between these...