By finishing the story if you didn't spend too much you will have around 200k gils (some will have more than 300k).
Specially if you take the allagan coins to sell instead of gear as quest rewards.

Then you will do dungeons, duty roulette, leves... this will give you some cash.
Depending how much time you play this is a correct source of income.

To be very rich only finding/making some stuff to sell (materia, materials for craft, crafted material, gear ...)