I just updated my personal sheet tracking these things. The parameters of that one:
- flat price 0.005 assumed until Mt.Gox opens
- for 7/2010-2012, Mt.Gox prices used
- for 2013, three exchanges
- for 2014, Bitstamp only
- daily VWAP
- first day 2009-1-3, the first day of Bitcoin; last day 2014-7-9, the 2014th day of Bitcoin
The exponential trendline for this time:
USD/BTC = exp(-2.869800 + 0.003012 * D), D being the number of days
R^2 = 0.934183
The trendline is at $1583.
Thanks for the update!
This version of the formula directly works on Wolfram Alpha (copy/paste):
10^(-2.869800 + 0.003012 * ((number of days since 2009 Jan 03)/days))
or just follow this link:
http://www.wolframalpha.com/input/?i=10%5E%28-2.869800+%2B+0.003012+*+%28%28number+of+days+since+2009+Jan+03%29%2Fdays%29%29