I've tested how soon can I do a manafarm which will have some reasonable self-payment time - my lowest level succesfull try was at lv73 - I've specially started a new game for it.
I've made a fairly weak manafarm at lvl 60. It only generated 57k mana. On the other hand I pulled it off with no orange gem skills or anything aiding them from talismans, on B2, with all traits (including mana lock) set to 7. Gained 250k XP as a result.