After unlocking new items for sale in each of the shops, I find myself constantly short on money. The merchant is somewhat helpful in farming money, but I'm trying to find the most effective way.
From my limited experience, what I've been able to make the most use of is Tressa as a scholar (for Evasive Maneuvers support skill). Then, walk around between very high level areas, running from every encounter if you find any (using the cleric run away support skill). You find big chunks of money faster in high level areas.