Why my coins cost 25% less than NPC's coins? It's a bit weird, each coin's cost should be fixed for everyone, it's a currency after all. I understand why my items cost much less than NPC's items, but I don't understand why it also applies to coins, it's stupid. Is there a mod that fixes that by any chance? Or maybe I just don't understand how it works? It also goes against the description of the large coin. It says "worth 10 coins", but you can only get 7 from one large coin. Am I missing something?
The mechanic exists to reference NPC affinities, personal biases, greed, and attitudes. Due to how bleak and awful the setting is and its people are, no one wants to spend 1-for-1 fairly and are thus all trying to scam one another to some extent, so depending on the NPC their personal trade value for coins varies greatly. For example, aristocrats have obscenely massive trade point values for coins because they're greedy and don't want to part with their money. Certain questlines for certain NPCs who are vendors will result in lowering the trade point value of their goods, including coins, because they like you and thus are giving you more fair pricing. Another thing is that everyone is specifically "Bartering", so most services aren't fully paid in coins but also other items and/or favors.
The bank actually has the closest thing to a 1-to-1 value for all coins, which shows the real value of the currency. Small coppers at their baseline is 200tp, large coppers 2k, and gold pieces 20k. The bank also is the best place to sell stuff for money due to having the fairest rates on bartering. When using the bank's rates, Lona's money and goods are only at a 10% devaluing (180tp for small coppers, for example), which shows how much everyone else is trying to be ripoffs as a neat worldbuilding detail.
So the 10 large coppers for only 7 small coppers is the result of whoever you're trading with scamming Lona to benefit themself for a 3 small copper profit away from Lona. If you want to convert smaller coins into higher value ones, always go to the bank.