Not to sound like jerk, but honestly, why does it matter?

The point of making it cost gil, is to lower the reliance on using the spell, and instead try out exploring or using chocobo porters. Using teleport comes at a cost. Because its a faster mode of transportation, it's gonna have cost(even more so come later in game when teleport costs get higher).

The idea of using an item that can be mined and sold on the market board is a bad idea, because now you're allowing players to control other player's ability to teleport. As you see with many items on MB, the cost of such an item would be ridiculously high. So to avoid it, players would in turn spend gil on the cheap, yet still expensive, item from vendors, sinking large amounts of gil into that way. Anyway you look at it, it would be a MUCH greater money sink then it is now.