The prices are different, and proprietary, because Coinbase is not an exchange.
They are a broker.
They purchase the coins, and sell internally. This is how they make some purchases "immmediately available" when they haven't even pulled your money yet.
They buy at X price, then sell to you at X+X price, to make a profit on their purchase.
As for the comment above that Coinbase is for noobs who don't use other exchanges ... can you clarify that?
Are you saying that experienced users prefer to spend $40 to do international wires and wait a week for the money to arrive, hoping and praying that the price hasn't changed $200 by that point?
Actually, I usually use a broker myself. Turns out wiring money to an exchange is just as fast, and I avoid the 8% fee - and can decide precisely when the right time to buy in is. Also found a localbitcoin guy who sells at stamp price. So yeah, brokers are for people who haven't discovered better methods yet. Never again.
