Interesting quote from that Gamasutra piece that was posted earlier:
“I will always wonder; given how much it has taken off, you have to wonder. You look at the amount of downloads and you go 'Wow, that would have made a lot of money if we'd had even a tenth of that in paid sales,'” Hagewood tells me. “But we knew what we were doing when we made that deal. We knew that was a possibility."
"I'm a very patient person, and I'd rather build this brand and make it become a thing than make piles of cash. I look at it as a long-term strategy: even if we have to wait until we make Rocket League 2, if it's become a thing, that's the most important thing for me. That it becomes this phenomenon. That people realize we've been making this really cool game, and now everyone's playing it.”
http://www.gamasutra.com/view/news/...re_worth_revisiting_A_Rocket_League_story.php
So it sounds to me like they don't get paid per download, but rather perhaps an up-front sum (varying from deal to deal)?