Prices on Mt. Gox tend to be higher than the prices on other dollar-based exchanges. The spread may reflect the fact that Mt. Gox users have struggled to withdraw dollars from the Japan-based exchange.
Interestingly, the price gap between Mt. Gox and the second most popular exchange, Bitstamp, has narrowed somewhat in recent weeks. At publication time, bitcoins were selling for $254 on Bitstamp, a 5 percent difference.