Just dump both slow and fast learner. It's easier. Characters who 'learn' slowly, and the player, for some reason, really wants to reflect that, can reflect this ability by spending XP on something other than skills, or just don't spend it if they really want to play a character inferior to the other inhabitants of the game. No one wants to take slow learner, and it makes no sense for any character who ever thinks they may spend more than 3k points on XP not to take it. In effect, not taking fast leaner is an XP 'tax' on characters with over 3k in XP spent on skills, and slow learner is just an even heavier 'tax' rate.
Don't adjust the skill costs, especially on attributes. Let skills cost what they should. It keeps characters closer to an equal footing. in terms of the impact of XP awarded over the length of a campaign. And it is a lot less work and headache for the players and the GM, and has no detrimental impact on gameplay, game balance, or character creation.