What I would like to know is how much money has actually been transferred into the exchanges. That is what matters. If Coinbase has 12 million accounts, and on average only $100 was invested (I'm assuming there are MANY opened yet unfunded accounts), then we're talking about $1.2billion at play moving back and forth on that exchange. (this is just a hypothetical number, I would love Coinbase to produce statistics, but know they have no incentive to.)
If no new accounts get opened and no more money flows in, That $1.2b will slowly drop to zero as Coinbase and the miners eat up that pool with fees.
No one really notes the "total volume" of the trades of stocks, which is what you're asking for for Bitcoin; it isn't really a meaningful number. If people pass a stock - or a Bitcoin - back and forth for 10 trades for [$99, $101] or for 100 trades for [$99, $101], it isn't really meaningful.
The best you can do for Bitcoin is to try to gauge the "depth of the market" right now, ie, how many "buy" and "sell" orders are out at what prices, how much Bitcoin you could buy or sell right now and what the average price you could buy or sell a large quantity of Bitcoin at. (Eg, if you wanted to sell X Bitcoin, you might be able to sell X/2 Bitcoin at $PRICE and X/2 Bitcoin at $PRICE/2, for an average price of $PRICE*3/4.)
OP is asking about Market Cap not volume.
The question at hand is - market cap vs. money invested by current owners of bitcoin. Hypothesis being that this ratio is high.
That total is almost certainly less than the "market cap".
What does that number tell you? It doesn't tell you the minimum value of a Bitcoin.