There's a site called Guild Wars Guru where players can auction their items. It's the most...reliable factor, since there's no in-game indication for the worth of items.
And any random drop can fall down. Elementalist runes from Warrior bosses, Daggers whilst you are an Mesmer, etc.
Anyways, the Furious upgrade is pretty rare. If it's the 'maximum' version, (most percent change of succes) it might be worth a lot.