Personally I don't like to not appreciate the character's statistics when a player put effort into having high Charisma. So I use this:
I like to think that when a player or NPC is persuading another one, it's because the other one has an idea of its own that is opposed or different from its counterpart. So I have the two characters make contested Persuasion checks.
Example:
- Player saying: "This item is not this expensive, lower the price!".
- NPC mind: "Mmm, I'm not sure, I believe this item costs exactly this price".
- Players rolls Persuasion to impose his idea.
- NPC rolls persuasion to preserve his idea. It's persuading itself.
