Well, the most consistent money makers are crafting 2 star ingredients that maxed out crafters will use in their 3 and 4 star recipes, and echatl sealants from getting ixali oaknots by doing the dailies for the ixali beastmen faction. Aside from that, gathering mats don't sell super well since there are literally dozens of people on each server farming stuff and putting it up on the board. On Gilg, Darksteel ore is cheaper to purchase than some of the lower tier stuff, even HQ ore. There's probably a decent amount of money to be made when the expac goes live by supplying the guys PLing their crafting right out the gate with the new mats mats, and I would expect prices to be pretty retarded on the new gathered stuff. But since the game has been out for awhile with only a couple of gathering tiers added, the standard stuff available just isn't a big cash cow.
If you can gather and craft the lower tier mats (check your board for prices) then you can make a pretty decent chunk of money that way, but you'll have to do a lot of it to make a reasonable return. The people making big money are the ones with maxed crafters selling... to other crafters. Materia has kept a lot of its value, and if you aren't doing the relic quest then you should dump all of it onto the auction board. Otherwise, just do challenges and if you dig crafting do the ixali dailies, though if you haven't started them already that's easily a couple of weeks to get to the point where it's profitable, and this close to the expac may simply not be worth it. Otherwise it's a pretty secure 60k+ each day for doing 30 minutes worth of dailies. But gathering? Sadly, it just isn't what it used to be.