Could someone explain me what sum of XEM should be profitable for harvesting?
What is profitable for you?
To get XEM from harvesting every week...
If you harvest with 1.000.000 Million perhaps you get 2-3 blocks per week.
I don't think you can call harvesting profitable, there are still too few transactions to give harvesting that attribute.
Supernodes on the other hand are profitable for sure but you need a lot of xem to join the program.