Mercing (selling runs) pays very well. You can make millions every hour, depending on the amount of customers and competition.
Crafting has pretty low margins at the moment unfortunately.
You could always farm materia.
Outside of that, you can do the general "broker" way and buy low/sell high. Watch patterns and see when certain items become more expensive (prices tend to increase on weekends, for example). Try to predict what the market will be doing in the future (due to things such as new patches or events) and act accordingly.