How much does Charisma impact buy/sell prices to vendors?

[Jingleson]

I'm going to make a Bard on Red99 to play during off-peak hours where I cannot get a group to grind with. I'm on the fence about whether I want to dump into Sta or into Cha. I'm leaning towards Cha because it'll help with Mezzing, but I want to know how much of a difference Cha makes as far as buying/selling prices with NPCs.

How big of a difference is there between 75 Cha and 100 Cha?