By season 100 I'm often gaining between 16-24 mana a turn. Add in that I don't 'waste' my mana in combat and only use it when necessary, I'm usually sitting on quite the stockpile.
Hmm thinking about this, while I use my mana (actually used alchemy once today, like wow 100 gold
). You might be right at the exchange, I don't mind alchemy and it should in no way be nerfed badly, mostly because while it might be powerful to convert mana to gold, its also some of the fun stuff in the game 
a 100 mana into 75 gold is still ok, I don't mind, I only use alchemy when I have too much mana and need gold, and I guess so would anyone else.
Sincerely
~ Kongdej