Leadership reduces the base cost by 5% per skill level. But the real cost is reduced by a larger percentage for every skill level. Let's say we have a theoritical unit with a wage of 100 denars. At level 9 Leadership, that unit's wage is 55 denars. By going from 9 to 10 Leadership, the wage goes from 55 to 50, which is a little over a 9% reduction.
The inverse is true for most skills that increase bonuses per skill level. The real percentile increase becomes lower as your skill gets higher. This doesn't mean that you should never max out such a skill, because even the diminishing returns may still be worth it, and other factors may offset this effect (such as the soak factor of armour in the case of Power * skills). But it's nonetheless useful to keep in mind that skills that reduce penalties become more effective at higher levels.
The inverse is true for most skills that increase bonuses per skill level. The real percentile increase becomes lower as your skill gets higher. This doesn't mean that you should never max out such a skill, because even the diminishing returns may still be worth it, and other factors may offset this effect (such as the soak factor of armour in the case of Power * skills). But it's nonetheless useful to keep in mind that skills that reduce penalties become more effective at higher levels.