The NFL pays for 150 rings for the winning team, putting limits on what they'll spend per ring. If a team wants additional rings, it pays for them.
The spending limits don't actually affect the ring designs much, because jewelry companies compete heavily to be chosen as the ring provider, sometimes offering the rings at a heavily discounted price in order to be awarded the prestigious contract.
The following pages give you a glimpse of Super Bowl rings from various years. Many of the rings were made by Jostens, the same company your own class ring or sports ring might have come from. Through 2004, Jostens had created twenty-five Super Bowl rings. Photos used with permission.