All the US dollar exchanges with substantial volume are within 1% of each other. [1] Some of the little guys are a few points out there, but any significant order would blow the price. All the exchanges lock together within the range of the cost of trading, which is usually at least 1% when you combine buying, selling, short-selling loan, blockchain reward, and cash withdrawal fees.
[1] http://bitcoincharts.com/markets/currency/USD.html