Crafting may not make as much income as some people would like it to, but it's a good way to save money.

Anapingofness mentioned taking the allagan coins as quest rewards, but that only becomes worthwhile if you have some other cheap way of acquiring gear. Taking a 500 gil coin rather than a helmet doesn't help if you spend 1000 gil to buy that same helmet. The standard NPC price of gear is nearly always higher than the amount you'd get by taking the allagan coin alternative as a quest reward. Market board prices are usually higher as well, although it's worth checking periodically to see whether there happens to be a particularly cheap deal on anything you need (or will soon need).

The way to be sure of getting cheap gear is to make it yourself from materials you gather yourself. That way you (1) don't have many expenses; (2) can freely take the allagan coins from quests so make some money there; and (3) can make some money by selling extra pieces you make. The combination is more powerful than just the #3 that most people look at when considering what crafting earns them.