The amount of gold that it takes to improve a vendor's attitude toward you increases with your level. It's only 600 gold early in the game. The BG3 Wiki has a breakdown of what it costs to increase it to +100 at each character level
here.
By act three, it will be thousands of gold. It's really only worth it if it's a trader who will stay around in future acts (like Dammon) and you expect to buy a lot from them.
