Leveling crafting classes (in a reasonable time frame) is most likely a money losing venture. At best you can break even crafting some extras for the quests you're doing once you have a few cross class skills.
Once you start to go the master crafter route and start melding materia, it's an extremely deep money pit. Think 30k per shot for a 20%-30% chance of success times 5 slots PER CHARACTER. It's insanity. I lost 8 in a row once... such pain. For the off hand tool, that shot costs 200k since it's a tier IV materia. I haven't even attempted it yet for any of my classes.
After you get through that, you might see some better returns, but so far I'm making more of low level HQ stuff than I would make on the 1-star or 2-star items I have available to me.
Desynth my actually be a net positive if you sold the demimaterias you get. But since you'll need them to unlock the new master books and craft the new earrings, choker and belt, you won't. It is a lot less expensive than melding so far, but as all things crafting related, I've just barely scratched the surface.