You wouldn't build one from scratch. You'd buy 1-2 to complete a weapon you already have sitting around. Even if you've played from the start and maxed every weapon, by the time this releases you'll *just* be completing a grid. But not everyone has been playing from Day 1, and not everyone has maxed every weapon for every element. Raid events are a pain to farm, after all, and they were even worse only a few months ago.
As for Orihalcum costs, if you're only farming for 1 element, based on my calculations you can easily spare the Orihalcum to trade for Raid weapons.