Yeah that was the issue, it was the best deal among teams willing to give him that big extension.
Did Oakland overpay him? Sure. But that always gets a bad label, and it's not necessarily a bad thing. They have guys on rookie contracts, and 4 of the first like 34 picks in the draft, so they can afford to overpay for guys. They might sign Bell too, and overpay him. But they'll have them. Then in 3 years, when they have to start paying their guys, they'll cut Bell and Brown. No harm done.