I’m not sure why he overreacts to the price of the skill books. He had just seen that healing potions were more expensive than expected, and that skill books were only available on the black market. And then, they turn out to be only double the price from the game. Then, he shows obvious reactions to the skill book he wants which lets the shop keeper charge him whatever she wants.

She said it was rare and that only an oddball would want it, which is like saying that it will never sell. There’s tons of room for negotiation.

Imagine a world where he wasn’t a complete moron, and he saw the book, and then calmly asked her about it. He could negotiate the price down instead of up.