easiest way I found to calculate the correct amount is to go to a converter such as:
http://preev.com/Type in 1USD and it chugs out the correct Bitcoin amount.
divide that number by the current rate of Nobl on
https://www.coinmarket.io/takes about a minute
[/b]
The site you mention is currently giving a BTC price of $893 while Bitstamp has it at $824.5

different sites have different rates based on the trades that are taking place within their exchanges.
for example: Bitcoin (BTC) Crypto Currency Current Bitcoin Price: MtGox Exchanage MtGox: $1,006.01 Coinbase Exchanage Coinbase: $828.77 BTCe Exchanage BTCe: $826.00 Vircurex Exchanage Vircurex: $800.00
each exchange posts the price of bitcoin based on the trades that are going on within each exchange
as far as valuation... what i do is add them all together and divide by 4 to get a rough average