Ancient coin mana sustain and indicator

So ancient coin doesn't have anymore mana regen and the only mana you get are from coins.

I feel like I'm running oom when using it, but I'm not sure how much mana I get from it compared to spelltheifs, or if I'd run oom if I had spellthiefs anyway.

What do you guys think of a note / tooltip that says how much mana ancient coin has earned total? Do you feel ancient coin line gives more or less mana then the spelltheif line?