Yeah, spiritbonding is probably the easiest way to make money without drastically changing how you play the game. You can certainly make a decent amount of money crafting (and gathering, but some materials can't be gathered more often than every 72 RL minutes), but the initial investment is enormous since you'd need all crafts at 50, lots of materia to outfit at least one of them, probably one or both gathering classes (fishing doesn't really count) decently outfitted, and you'd need to make your way through the artisan grind. It's a lot to do if you don't actually want to craft (or gather) and just want to make gil.

As far as spiritbonding goes, crafting and gathering materia prices are close to what you might expect. They get a bit more expensive every tier, with tier I actually being pretty expensive because the demand is very high (you use several tier I's overmelding crafting/ gathering gear). Tier IV are pretty expensive. Battle materia are a bit all over the place with pricing due to Novus and the fact that several varieties are useless or near enough. Also, because several people are actively trying to get Savage Might IV, for example, many of the tier III materia are cheaper than you'd expect (their supply is higher than the supply of tier II materia). Because of this, spiritbonding battle materia can have more varied results. You can be unlucky and get nothing worthwhile (all elemental resist) or very lucky and make a fortune (lots of red tier IVs). Crafting/ gathering materia is a lot less random. Less likely to win big, but less likely to lose.