Saying on average a Stone II Would do ~1.5k damage, that makes out to be 30ish MP return, making tier 2 spells free when using this.
Also if you have Mana Wall up and are falling low on MP cast stone a few times and you can keep yourself up a little bit longer.
You cannot recover more than the spell's base mp cost. Also no mp recovered during manafont manawell. Use this for Aja and aga spells or if mp is extremely low.