The general consensus is that 6%-9% of root bark (I am assuming by weight here...correct if wrong) is sassafras oil. So, if you have 1 lb of bark, that 16 oz. There are 28 g per ounce, so there are 448 g in a pound. 6%-9% of that is 26.88 - 40.32 g of sassafras oil. Depending on the tree you got the bark from, the safrole content in that oil will be 80%-90%.... I always like to use conservative estimates, so we'll say your oil has 80% safrole. 80% of 26.88 - 40.32 g is 21.50 - 32.27 g of safrole per pound of root bark.
You also must factor in the freshness of the root bark, because the fresher the bark, the more oil you will get out of it. I remember reading a post (can't remember by who, sorry) but they said that they only got 1 g of safrole from steam distillation of 1 kg (yes, that's KG) of old root bark.
So, to answer your question, you will need 8 - 9 pounds of fresh root bark to run a Dr. Drool synth.
An alternative to this would be buying live sassafras trees from a nursery or a certain popular auction site (there are a bunch of trees for auction at this very moment...and they are cheap)... or go into nature and find your own damn trees and cut them down. It's actually a fun thing to do, plus you learn to name a whole bunch of trees.
Eventually, you can get cocky with your friends and tell them that you bet them $100 that you can make mdma from a tree. They will of course take the bet, and you will win a $100. Of course, if you are stupid enough to tell your friends about your batcave activity, you will also probably win yourself a 20 year vacation to the new Bally's Resort on an "island" named Clink.