Gil

is the currency of the entirety of the shard of Hydaleyn. There are many ways for adventurers to earn Gil, some of which involve slaying monsters, completing quests, and selling items and equipment.

Cost

To keep things consistent with the D&D 5e Players Handbook, you can use its prices but convert them. Gold will take the 100s place, Silver 10s, and Copper the 1's. For example, if a potion costs 50 Gold Pieces then it will equate to 5000 Gil. If a basket costs 4 SP then it will cost 40 Gil. If a pitcher of ale costs 2 CP then it will be 2 Gil and so on.

Allagan Tomestones

They are solid-state data archives used by the Allagan Empire, a civilization that thrived during the Third Astral Era, and as such are considered very valuable. There are numerous organizations, scientists, and historians that would love to collect this and would reward you handsomely. Rowena is the most notable figure that collects these and would provide adventurers with magically enchanted gear in exchange.