are u sure u dont mean 3000 gil cause 300 gil is a bit cheap and a lantern sheild isnt that expensive now having leaping boots and a emperors hairpin is
it all depends on the application. If you are a thf or a nin, then you'll probably be better off with the compound eye circlet, but if ur a pld, or a war with a sheild, then you're better off using this.
You buy it with Conquest Points from the same guys you get the signet from (at least in San d'Oria, it's a T.K.). I don't know if it's possible to synth the "Royal" anything.