Is recruiting heroes $ amount based on my sovereign's charisma or the person doing the recruiting?

Sovereign's charisma is the only thing that matters.  Unless it's changed in 1.09 champion charisma is worthless.

Does charisma effect anything other than recruiting cost?

That's good to know.  Wondr why they would go into such detail about such a worthless function?

The attribute system needs to be fleshed out a bit; right now it's not doing very much, and we may as well just rename strength and dexterity to attack and defense.

Bards have an effect on costs, I've noticed that my hero recruiting costs dropped drastically after getting 3 bards towad endgame. I think all the hero benefits are global, as I've noticed the reduced cost even when other guys did the recruiting, in fact, at some point in the game you don't even have to be adjacent to a recruitable unit to recruit it. I think this is tied to some tech advance, I don't know for sure as the manual is incomplete & doesn't cover most of the game details.

You can recruit those in you zone of control even if you do not have someone next to them. Early in the game you are more likely to find them outside the ZoC than in the late game.