User blog comment:R E W S/The Best champion in the League of Legends:Arcturus, The Stellar Morpher (FINISHED)/@comment-64.25.232.226-20121203150002/@comment-79.217.81.52-20121203172838

You're right. You need 500 Mana to regenerate the cost of the spells with the passive. Without any item you hit that mark at level 6.