FBI's way of estimating "1.2 billion" involved pricing the net turn-over in Bitcoin (over the course of silk roads existence) at it's highest values in order to give the news a sensational effect.
Silkroad is a joke compared for example to some 300% increase in heroin production that Afghanistan saw after America invaded - it's just the later isn't favorable to publicize.
Notice - Even wikipedia blatantly quotes the misinformation published in the document in it's article:
It noted that, "From February 6, 2011 to July 23, 2013...
The total revenue generated from these sales was 9,519,664 Bitcoins, and the total commissions collected by Silk Road from the sales amounted to 614,305 Bitcoins. These figures are equivalent to roughly $1.2 billion in revenue and $79.8 million in commissions.
But - for the bitcoins to amount to 1.2 billion dollars they would have to be priced at a relatively high 2013 mark at least 100 magnitudes above 2011 prices (from which their estimate begins).
For the record the world-drug market is estimated at around $400 billion, so if silkroad over the course of 2.5 years had a turn over of 213 million, that would place it as accounting for .02% of the drug trade. Not the staggering blackmarket the FBI and US media have made it out to be.