Opportunity cost is important for a crafting business. However, many items are not produced by crafting businesses.

Instead, items are waste products. The low level crafting economy is mostly a service that sells crafting xp. Players purchase raw supplies to level crafting. They dump crafted goods. They mine for xp and dump ore.

Dumped goods compete with businesses. Businesses can't compete without profits, so they close.

The xp service economy depends on new capital and new players to produce items, because xp is the primary profit from producing items. Items are worth even less gil than the OP suggests: they have no gil value, not even repair costs. Severe undercutting will remain rampant.

The goods have some value to the players who want to buy them, but it would take a real business to find a competitive and sustainable price. That won't happen if the market remains saturated with dumped goods.