Silk Road uses a 24 hour rolling average to determine the value of Bitcoin on the site. This is done to protect users from price fluctuations, so instead of being subject to a large loss if Bitcoin value suddenly drops by 10 -15% (as would be the case if Silk Road used the 'last' BTC price) members are insulated from the volatility. Given Bitcoin's volatility you should always deposit a little more than you think you're going to need for your purchase. Libertas