Okay, I may have missed it and can't find it under search but...I don't know what the drop rate is for zetas on the challenges Let's just say the drop rate is 50%. Which I would consider high for E...
I am taking the challenge approach and not the shipment approach. This game is all about the long run, and in the long run I will be better off farming ships and being able to farm the challenges than buying one zeta mat every other day.
Plus, we all know ships will be more important later on. So farming them will help for that eventuality too.