I believe the $80mil estimate is from the criminal complaint, which derived that estimate from the data they collected from the seized server. The complaint specified that from Feb 6, 2011 to July 23, 2013, Ulbricht collected 614,305 Bitcoins in commission, which is $84,159,785 at today's exchange rate on Mt. Gox.
So I'm pretty certain that the $80mil estimate is derived from the actual number of Bitcoins in commission that was logged on SR's server.
Calculating (volume x last price) can be misleading when the number of coins traded is large enough to move the market.
Using the freenode/#bitcoin 'gribble' irc bot for some calculations, there are only 45,000 bitcoins or $5Million for sale over $100USD.
donpdonp> bids 100
gribble> There are currently 45669.225 bitcoins demanded at or over 100.0 USD, worth 5307618.93329 USD in total.
Selling the remaining 550,000-odd coins would wipe out the market and net only $6million more.
donpdonp> market sell 600000
gribble> A market order to sell 600000 bitcoins right now would net 11853320.3682 USD and would take the last price down to 0.0100 USD, resulting in an average price of 19.7555 USD/BTC.
For those who are claiming that this is a setup, that he is innocent, well... he's certainly got to face trial. But he's going to have a hell of a time arguing against this evidence, if even one planted FBI/DEA 'controlled buy' can be traced to this account (handy thing, the Bitcoin trail... unless you're trying to disavow knowledge - it's probably going to be hard for his lawyer, who claims that Ross is "innocent of all charges and this is all a mistake".
They are claiming innocence but the burden of proof is beyond reasonable doubt. What his lawyer claims in public is irrelevant to the trial.
DPR has a number of defences available that could render the issue of the wallet moot. Most importantly by denying access to the wallet he has a huge point of leverage, both in settlement and retaining counsel. One of the keys to drug law is seizing all assets so that the defendant has no means of defending themselves.
Perhaps the settlement is no prison time and a $80 million dollar fine. Keep in mind that in order for it to be introduced as evidence a copy must be provided.
So I'm pretty certain that the $80mil estimate is derived from the actual number of Bitcoins in commission that was logged on SR's server.