The robes cost like 60k tokens, which takes 37 hours to earn if you play 50/50 games. If you assume runespan is 80k xp/hr (I don't even know the rates) earning the robes makes you waste about 3m xp. That means if you earn at least 60m xp (5% of which is 3m bonus) using the robes, you have earned back that xp and any extra xp is absolutely free. In short, unless you have 137m or more xp, the robes are worth getting.
Hi
04-Oct-2013 13:36:29