Who was standing at the market? I sold all the remaining minerals on the last turn.
Basically, I just checked – I have: 3 sulfur (450 coins), 5 mercury (750 coins), a bunch of gold (900 coins), and a chest (1000 coins). In total, 3100. Minus one hero (2500 gold). That's exactly a 600 coin difference.
The starting hero was at the market.
I guess I'm just used to playing at a fast pace.