The only limiting factor to Nasus' Q farm past level 5 is items. If he hasn't gone back for Philo/Sheen/Something-with-mana then he will run out very quickly, otherwise he can generally sustain for a very, very long time.
Refunding the mana cost would literally make Nasus impossible to remove from lane, as he would just have to build high defense in order to make it impossible to get him away from his farm. No mana items required, and his lifesteal gives him healing for free.
Originally Posted by Warrikon
it just works with his kit :\ dunno why they haven't made it this way. I get why veigar and sion do this because their spells actually get significantly stronger but nasus just gets cooldown and i think +100 damage from it.
And every time he uses it properly it hits for 3 more damage. Getting levels in it should ensure he can clear an entire creep wave with it. That is 6 last hits, or 18 damage, plus gold from that. That is a really big deal. That is almost two free Long Swords worth 800 gold. And instead of spending any he actually gains
Really shouldn't compare Sion to this, by the way. He is old, broken, and has terrible mechanics and skill synergy. He is either an AP nuker with autoattack spells or an AD carry with minor stun and shield. And his passive is the only RNG left in this game that is still totally borked. He is in dire need of a recreation.