Well, it turns out that the NFL paid Mars what it pays all of its Superbowl halftime performers. Nothing! Not a penny.
You see, more than 100 million people are watching. That's his payment. Last year's performance featured Beyonce and Destiny's Child. Following their 12 minutes of NFL fame, their album sales increased by 40%. When The Who performed, sales of their Greatest Hits album more than doubled.
It's exposure. It's the same reason why advertisers spend millions to hawk their wares.