When purchasing an item through BitPay, how do their exchange rates compare with the market exchange rates? BitPay allows merchants to price their products in USD, and receive USD when a customer pays using Bitcoin. The exchange rate used at the time of purchase is calculated somehow by BitPay, but their website doesn't indicate how it's calculated.

If at the time of purchase BitPay immediately sold an equivalent number of bitcoins on, say, MtGox then the exchange rate used should be pretty close to the highest Bid price at that point in time.

Many people must have purchased items through BitPay. I'm interested in knowing what exchange rates were available on MtGox (or other exchanges) within a few minutes of the time of purchase, and what exchange rate was provided by BitPay. Can anyone provide an example of this?
Their website does have the methodology by which the exchange rates are calculated.