How does Charisma affect itemcost? I can´t seem to find any charts on how the cost of items go up or down according to Charisma... Am I stupid???

no. Item cost is dependent on reputation. Not charisma. Reaction will depend on your charisma. The better your reaction the higher rewards can be, but it doesnt affect item cost.

I got this from the BG2 game guide.

Reputation Discount
20 -50%
19 -40%
18 -30%
17 -20%
16 -10%
15 -10%
14 Normal
13 Normal
12 Normal
11 Normal
10 Normal
9 10%
8 20%
7 20%
6 30%
5 40%
4 50%
3 100%
2 the store doesn't sell you nothing

Actually, CHA can affect the cost of items, although I've never found a table that shows the impact. Just try casting Friends, and see how the price drops.

Note: I believe that it's the CHA of the person in the lead spot that is important here, even if that person isn't the one doing the talking. I tend to get the best results with having the lead person being someone who can cast friends, and have that person talk to the vendor. However, even then, sometimes the price drops, and sometimes it doesn't - I have no idea why, though.

I believe CHA affects item cost only if it is higher than the shopkeeper's. If it's lower, no effect.

(I think it's logical. Only the most charismatic people can become the most successful traders. Get lower buying price, higher selling price, all that adds up, and the person can achieve success sooner than their less charismatic friends.)

Now that actually makes sense. It seems to me that I only get discounts for raising CHA to very high levels; if it only goes from, say, 8 to 14, then usually no discount, but if goes from 18 to 24, discounts kick in.

Hmmm..... Just curious, I guess. I´m not going to lose sleep not knowing the exact numbers of such a chart, if it exists. Thanks anyway.

well you should always have a reputation score round 16-18(depending on the npc's you have in your party) and always use the npc with the highest charisma to speak with the storekeepers in order to have a good reaction

Well, I make a point to equip all the CHA boosting stuff, before doing the potion swap trick with a scroll of Friends...mostly because I want a max CHA of 25...

With a reputation of 10, and a CHA of 18, Girdle of Hill Giant Strength costs 10900 gp, but with a rep. of 20 and CHA of 25, it only costs 6600.

Both charisma and reputation affect prices.