Just why is Katy Perry performing at Starkey hearing gala in St. Paul Sunday?

Katy Perry at the Super Bowl/ Associated Press photo by Michael Conroy

How did the Starkey Hearing Foundation of Eden Prairie land Katy Perry for her only U.S. performance this year – besides, of course, the Super Bowl?

The Starkey people asked her people. Duh.

OK, it's not that simple to secure one of the hottest pop stars – Forbes' highest paid entertainer in the world for 2015 – to play a 1,600-person fundraiser Sunday at St. Paul's RiverCentre ballroom, where the Starkey folks hope to top last year's record of $8.7 million.

"It's a real coup for the charity," said Gary Bongiovanni, editor of the concert trade journal Pollstar. "She's not doing it for the paycheck. It has to be something she wants to do. Maybe she has a family who has a tie to the cause."

Or maybe Perry sees Starkey's mission – outfitting children in Third World countries with free hearing aids – as an extension of her work with UNICEF.

In 2013, she was named UNICEF Goodwill Ambassador "with a special focus on engaging young people in the agency's work to improve the lives of the world's most vulnerable children and adolescents."

Whatever it is, it's unlikely that Perry will be seeing her typical pay day.

When she performs in arenas charging from $29 to $150 a ticket, she grosses a cool $2 million per night. She leaves the show with 60 to 65 percent of that money, Bongiovanni figures, but then she has to pay her expenses. That includes travel and hotel; musicians, crew and staff salaries, and the cost of the stage and lighting production, which is amortized over the course of a multi-month tour.

Still, she realizes a handsome sum.

In fact, Perry ended up grossing $135 million in 2015, according to Forbes. The only celebs to beat her were boxers Floyd Mayweather and Manny Pacquiao -- and they worked only one night. But that's a different story.

This year, Perry has performed in Europe and Asia and is headed to South America in September, but she has only two U.S. performances on her 2015 schedule – halftime of the Super Bowl seen by millions and the Starkey Hearing Foundation's 15^th annual So The World May Hear Gala for fewer than 2,000.

"They're probably not paying for Clinton and Bush to be there," Bongiovanni speculates even though command hefty fees for speaking engagements.

In the music business, here's how it typically works when stars play private gigs or benefits.

If it's a corporate show for, say, Google or Wells Fargo, a big star can command a bigger-than-usual fee. For example, if superstar XX typically receives $500,000 for an arena concert, she might ask $1 million for a corporate gig.

In 1997, Bob Dylan and his rock-star son Jakob Dylan both played at a private concert for Applied Materials, a semiconductor company in Santa Clara, Calif. – the only time they've ever shared the same bill (they didn't play together, however). They each reportedly received a healthy six-figure sum – far above their usual fees.

Nice gig if you can get it.

Some have a discounted charity rate, some accept a fee and then donate it back to the charity, some work for expenses – meaning first-class airfare and hotel accommodations plus wages for their musicians and staffs.

"Even if Katy Perry performs for free, her band wants to get paid," Bongiovanni said. "And the stage hands. There are legitimate hard costs."

We'll have to ask Katy Perry about it on Sunday. Or Starkey mastermind Bill Austin.