Making money becomes easier as you progress through the main story and approach the end game. You can acquire two Amulet Coins to double the prize money for beating trainers. However, the best way to make money in Pokemon Soulstones is to sell Fossils and Relics acquired by using the Mining Gear on rocks. Fossils have a high spawn rate in rocks; the best place to mine rocks in Pokemon Soulstones is the Carleton Mine and Diadem Mountain.