Coinbase pricing mystery

I noticed since Mt.Gox stopped allowing transfers for USD that the prices of Coinbase.com dropped. However, I can’t figure out what index Coinbase is using to determine pricing. Previously it was easy to figure out the price, Mt.Gox * 1% fee + 15 cents (bank transfer cost). It seems to me logical that they would be using Bitstamp, but really all other indexes are about ~10% lower than Mt.Gox. The new price for Coinbase seems to be Bitstamp*6% but that fluctuates. The question …