What is the most reliable way to calculate the amount of fiat value that has been transferred into the bitcoin economy? I would like to know what are the most sophisticated ways to estimate the amount of bitcoin that has been bought globally with various fiat currencies. Exchange data is only one source, and may well cover a large percentage of value that has been transferred but I can't help thinking there must be a lot of "off exchange" trades going on which may well account for a lot of value. I would also consider fiat money spent on mining investments including implied energy costs as a transfer of value.

I'm wondering what is the opinion of the community on the best way to measure how quickly fiat value is being transferred into bitcoin? 

To put it another way, what is the shared "stake" in bitcoin?
I'm pretty sure you'll never get an accurate number. I'm not even clear what you mean by transferred into the bitcoin economy.


If I bought $100 worth of bitcoins 3 years ago, do we count that as $100 or the value of the BTC now?
If I buy a bitcoin for $100 cash (using localbitcoins), send it to my friend (who immediately converts it to $100 USD), how do we account for that? If I do that transfer millions of times, can we say that BTC has "Millions in cash invested" even though there was only ever $100 at a time?


Your best bet is to just add up the aggregate transfers on all the exchanges, and call that "GDP" (and you may never be able to account for non-public transactions like localbitcoin or private exchanges). Just like GDP, you will necessarily over-count what's really happening. (For example, my transaction "I buy shoes" is actually counted as "I buy shoes, the shoe seller buys the shoes, the shoe maker buys leather, the leather maker buys a cow, the farmer buys cow food, etc."