Hi there. Actually, I did cover value of Bitcoin in use as a medium of exchange in footnote 2. Because there is no production in the bitcoin economy, or very little to be entirely accurate, the real exchange rate should basically be one. Why? Because most users of bitcoin live within very large production economies. Why would buyers pay more or sellers accept less, if they would be better of transacting in the domestic economy?

You might imagine a slight premium for bitcoins because they facilitate transactions that would be difficult in the domestic economies, such as money laundering or black market trade, but you could just as easily argue that the real exchange rate is less than one due to the relatively lower number of people to trade with, concerns regarding being hacked, and uncertainty regarding future government regulation of bitcoin. That's why I decided the most neutral approach would be to assume purchasing power parity across bitcoins. Admittedly, this assumption may not be great as speculation runs rampant and everyone runs for the exits, which would imply a heavy discount in the real exchange rate.