I think the price difference is normal between exchanges and there are different things acting together to cause this. Talking about bitfinex i think the volume they have is always strange and remain always high even for sure they have lost many of their real big traders recently after they got hacked.
Right now if you look at the chart, bitstamp is only behind 7$ and only <1$ behind btc-e. I don't care much the price each exchanger have, but security they have is main thing to consider and for sure i don't gonna trust bitfinex again.
There is a coindesk story entitled
Bitfinex to Hacker: Can We Have Our Bitcoin Back? that speculates the Bitfinex hack reduced the liquidity on exchanges, resulting in single massive trades pumping the price sharply on single exchanges.
For example, market analysts continue to assert that the massive reduction in market supply caused by the theft has had an adverse effect on interest and sentiment, while at the same time reducing exchange liquidity.
In the past few months, the bitcoin market has also provided evidence to these claims, rising and falling sharply on what analysts suggest were likely single outsized trades.
Bitstamp's price is often below Bitfinex's, but $20 is a bigger difference than normal. It's only $10 now, the $20 difference could have been caused by a few big buy orders in a thin Bitfinex order book.
That coindesk story also poses the question of how can Bitfinex be offering a bounty to the hacker to return the coins while it's still supposed to be working with the FBI to catch him. I thought the FBI always advises companies not to pay hackers.
