Triple Triad could also be a decent gil sink.

Have card shops in all 3 cities. Each shop sells starter packs and booster packs that feature NPCs and monsters from that region.

Starter Pack 1: 5,000 gil - Contains 10 random low level cards, and one random medium level card.

Booster Pack 1: 50,000 gil - Contains 5 random low to medium-high level cards.

Booster Pack 2: 150,000 gil - Contains 5 random medium to medium-high level cards, with a chance for 1-2 of them being high level cards.

The best cards have to be "Monster Carded" from high rank mobs, or won from a high rank NPC, or as tournament prizes.