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There are a variety of variables that affect Bitcoin pricing on the exchanges.
Here is some of the information that I have found on the topic as it was A question of mine to find out why its slightly different on each exchange.
Market size:
Relatively speaking, the market for Bitcoins is small. In April of 2013 it was about 1.2 billion USD, and a few days later dropped to below 750 million USD. That's a small market cap, which means, among other things, that there's less consensus on the price to the BTC. This is the capitalization for all mined bitcoins and includes coins that have been lost, so the real value is something smaller. Each exchange is a subset of the total market, so those markets are even smaller, which allows for greater variation.
Exchange Volume:
For all the coins that have been mined, the quotes are only from online exchanges, which are a small set of the total coins that have been mined. If only a quarter or less are in play, then the swings can be pretty dramatic. Since the volume is limited, and people don't take full advantage or arbitrage, different prices can and will exist on the different exchanges.
This happens with foreign currency exchanges too. However, with professional traders, billions of dollars, and serious automated trading the differences are in fractions of a percent.
Price of entry:
It's relatively cheap to enter the BTC market and cheap to trade. Furthermore, if you mined BTC when it was trading for fractions of a dollar, or even $30/BTC, there's no issue unloading it. With a low price of entry people are less serious about how the trade their BTC.
Fundamentally, BTC is a small, highly speculative, irrational market. Each exchange is a small, highly speculative, irrational market. That's why the spread is so great.
Here are some of the prices on some of the Exchanges that I use and it's all at the same time including google.
