Hello there!
I've been playing with a few new characters lately and tried out the Merchant class.
I can gain +1 to the Merchant skill when putting a point into the Charisma attribute and an additional skillpoint into the Merchant skill itself.
I can't make out the formula for the discount though. The values/percentages just don't add up. Does the overall sum of the Merchant skill only matters and there's a table based on that skill or is it calculated somehow?

WBC2 or 3?

WBC3

I just don't understand the math behind this Skill/Attribute. Can't see a pattern/formula for it. For instance:
I created 3 characters. All of them as Fey Merchants (starting with 6 Charisma and 3 Merchant) and leveled all of them to level 15
Hero 1 put all the points into Merchant:
- Charisma 6 (+1 Merchant (0,99% Discount))
- Merchant 17 (+25,37% Discount)
- Overall (bottom of the screen or Combat tab in hero info): Merchant 18 (15,25% discount)
Hero 2 put all the points into Charisma
Charisma 20 (+15 Merchant (13,04% Discount))
Merchant 3 (+5,66% Discount)
Overall: Merchant 18 (15,25% discount)
Hero 3 put points in such a way that Merchant from Charisma was the same as Merchant in Skills:
Charisma 19 (+14 Merchant (12,28% Discount))
Merchant 14 (+21,88% Discount)
Overall: Merchant +28 (21,88% discount)
Additionally I created:
Dwarf Merchant:
- Charisma 5 (+0 Merchant (0,00% Discount))
- Merchant 3 (+5,66% Discount)
- Overall: Merchant 3 (2,91% discount)
Dwarf Warrior (5 starting Charisma, no additional Merchant) and leveled him up to 13:
- Charisma 17 (+12 Merchant (10,71% Discount))
- Overall: Merchant 12 (10,71% discount)

That does seem strange ... good research, by the way. But what are you seeing for actual discounts with those heroes? Not what it displays, but actual cost of some nice pricey purchase? The proof is in the pudding, and all that.