Henry said:
In my campaigns, I always have the two hagglers compare diplomacy checks to gauge prices, and allow them to take 10 if they wish. Every point of difference between the rolls equates to about a 2% price difference, rounded off for ease of use. I don't tell what kind of a discount the palyers get, just like the fact that shopkeepers don't normally tell their customers how much of a discount they are getting from the cost. It's haggling, and it forces the players to have their characters get good on their negotiation skills, or get stiffed.
This sounds like a great system to me. After all, while charisma does figure into price negotiations, haggling skill is gonna be more important. (I'd give a +2 synergy bonus on this check for any character with 5 ranks in appraise, btw -- and you bet your britches the merchant's got those 5 ranks!)
[EDIT: Although most merchants will have those 5 ranks, high-level wizards are unlikely to have them. Indeed, some of these bookish NPCs may be so divorced from the real world that they have no idea what constitutes a fair price for the items they create. This will be reflected in their own low charismas and their lack of diplomacy or appraise skills.)
It sounds to me like you're penalizing the PC more than is appropriate: it sounds to me like more than a -1 penalty on Charisma checks. But I also think that a lot of the penalties for charisma ought to appear in normal gameplay, not just in costs.
For example:
-NPCs will look to the high-charisma people as the party leaders, will listen with respect to the high-charisma people, will tend to laugh off or ignore the input of the low-charisma people. When the fighter mentions that maybe the city guard ought to be ready for orc invasions, the guard captain will sneer a little, raise a conspiratorial eyebrow at the bard, and continue with their discussion as if the fighter hadn't said anything.
-NPCs will be quick to take offense at jibes by low-charisma people, not giving them the benefit of the doubt. When a CHA 17 person says, "Your daughter is as smart as she is lovely," the NPC will blush and thank them for the compliment; when a CHA 8 person says the same thing, the NPC will say, "Keep your greasy hands off my daughter!"
This should be subtle, and not overplayed.
Our party's "tank" (actually a monk/rogue, but the best fighter we have) has a six charisma, and the player plays it to the hilt. He's constantly grouchy and viciously insulting, even to his beloved sister. Those of us who know him laugh it off, but strangers that meet him are repelled by his terrible attitude. There are several merchants who refuse to have anything to do with him by this point; our party sorcerer surreptitiously buys things for him now. It's great fun for everyone.
Daniel