The demand in USD is about 1.9 million per month (http://arstechnica.com/tech-policy/2012/08/study-estimates-2-million-a-month-in-bitcoin-drug-sales/) which works out to be $63,333 per day (the demand)
There are currently 10118337 bit coins that have been created and each coin is spent on average once every 399.357 days (http://abe.bitcoinstats.org:2750/)
This means that each day there are 10118337 / 399 = 25,359 bitcoins available to use for transactions each day (the supply)
The price of 1 BTC should then be supply / demand = $63,333 / BTC 25,359 = $2.49 USD per coin
Does anyone have alternative suggestions for calculating the price?