Hi, judging from the league perks that encompass other perks and make them available to an entire league, the cost in slots would usually be the level of the other perk, plus one.
So, if a campaign empire was to allow players to do likewise for other perks, should the perks that are treated as league perks also cost their level plus one slot?
Of course, it could be best to limit this to level 1 perks, to prevent low-level characters from accessing perks which are normally beyond their reach...