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not exactly, your calculations too sharp, but if you are willing to do 350 tail run at once - then yes, but you are converting shrap to ammo, no? other stuff selling out no? so basically it depends on your stuff you are trying to get them and time
150 essence per piece
if you think you can get essence in a week without buying it in AH you are wrong :) actually you need to grind, sleep, work, toilet time, sexy time in essence place to make one piece updated in a week, if you are lucky enough
You are bit lazy aren't you? :) Tail tips drops, just they are not uncommon, looks like they are rare now. Sometimes you are getting them after 10 kills sometimes 2500+ kills
so the answer is:
e) Spawn is lying to you