They're paid for the same way you pay for eShop games.

From Nintendo Life:

StreetPass Squad - Side scrolling shooter, where you're an elite team member of the space police; your mission being to heroically save StreetPass Galaxy from the Gold Bone Gang.

StreetPass Garden - Become Master Gardener under the guidance of Mr. Mendel after you move to your new home in Leafington. To progress, you need to grow lots of flowers and decorate your garden with people you meet. Mii encounters are very important as it helps your plants to grow.

StreetPass Battle - A game of strategy and luck, you begin by ruling a remote country in a war-torn land. Wentworth, your trusty butler, helps you build an army via Streetpass to battle other generals. Your ultimate goal is to conquer the world in this quest

StreetPass Mansion - You're a detective sent on a spooky paranormal investigation - Luigi's Mansion: Dark Moon, anyone? - to solve the mysterious happenings at a haunted house with your young assistant, Ella Mentree. Reveal the truth, and battle your way to the top floor.