The T-Mobile price should be what all Starbucks hot spots cost, since Starbucks has contracted with T-Mobile to handle their wireless networking. That said, let me emphasize the "should" in the previous sentence. I have heard of places where something isn't working right in their server and people get a "24 hour test ride" any time they visit. I would also expect that places that are crammed full of wireless users may charge a premium over the basic cost to discourage a few users and thus keep from saturating their network.
The bottom line for me would be to expect to pay $6 an hour, be happy if I'm not charged, and keep my eyes open for something that says the cost is higher. It's their back yard and their ball, so you have to play by their rules.