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...
Basically, if zetas are on farm, and you're buying two zetas every three days finishing top 50 pvp, you should get roughly 40 zetas every 4 weeks, or ten zetas/wk. Some weeks more some weeks less, but avg 10/wk. For every zero drop, you'll have some double drops.
Zetas on farm basically double your accrual rate from just buying alone