This might sound a bit stupid but... I've seen a few teks for extraction and most of them use 500/1000g of rootbark. What would be the 'minimal' amount to obtain a decent result?

MHRB comes in at about a 2% yield and ACRB, I think, is a little weaker

50g of Mhrb will get you around 1g spice (around 40/50 hits)
Although as 3rdI says ACRB is a little less at around the 1-1.5% mark

(depends entirely on the quality of the bark of course)

I'm not sure about that, the less yield with ACRB......I'm on my ninth freaking pull on 100 gram extraction, and I keep getting more and more........I never got past 3 pulls with mimosa. Though I am certainly no expert with extractions, so I'd say 3rdI and Mr. Cyb know a hell of a lot more about it than I do. I think I just got a batch of bark from Jesus's personal garden of ACRB.