The way developers sell apps has changed. It’s no longer as simple as in-app payments, or a premium price to download. What comes next, though, is anything but settled. One promising path is Amazon Underground, a new model that’s grown exponentially since launching a few months ago.

For those unfamiliar with Underground—which, realistically, includes most people—the model is incredibly simple, especially for customers. Simply download a game, usually one that costs at least a dollar or two on other platforms, and play for free. There’s no up-front cost, and no in-app upsell. Amazon pays developers, meanwhile, based on how much time people spend using the apps.

But how much do they pay? And how much time do people really spend in them? Amazon has shed some light on Underground, and how quickly it has grown in a challenging climate. As with its hardware sales, the numbers Amazon shares aren’t absolute. Instead, they’re percentages, so it’s impossible to say exactly how Underground sizes up next to Google Play and the App Store. (Hint: Still very, very small!). The growth has been significant, though.

Royalties paid to developers are up 3,600 percent since Amazon Underground launched in August. They grew 50 percent from December to January alone. The number of developers on the platform has more than tripled since launch, and the customer base has grown 870 percent.

You can attribute much of that growth to the popularity of Amazon Fire tablets, which come with Underground pre-loaded. That’s especially important, because Underground can be tricky to access on devices beyond Amazon’s ecosystem. Android owners must contort to a multi-stage download process to install Underground on their phones; for obvious competitive reasons, Google doesn’t allow apps that sell apps and games in its Play Store. The iOS faithful, meanwhile, can’t access Underground at all.

You can also, though, attribute it to a system that benefits customers and developers equally. In-app purchases, after all, often are a scourge for both. While no one wants to stop to shell out a few bucks to advance a level, developers are also often loathe to interrupt fluid gameplay to ask for those shekels. It’s just been the best way to turn a profit. Or was, it turns out, until Amazon Underground.

Getting the Goat

Coffee Stain Studios is a small outfit, with seven games under its belt. The one you’ve heard of, though, is Goat Simulator, an app that delivers very pointedly on its title. You simulate being a goat. That’s basically it.

“Goat Simulator is a completely stupid game,” disclaims the company’s own description, “and to be honest, you should probably spend your money on something else, such as a hula hoop, a pile of bricks, or maybe pool your money together with your friends and buy a real goat.”

Its audience didn’t listen. In the nearly two years since its release, Goat Simulator has been downloaded over 500,000 times in the Google Play Store, at $5 a pop. Variations like Goat Simulator MMO Simulator and Goat Simulator GoatZ have each been downloaded over 50,000 times each, also for five dollars. The iOS App Store doesn’t offer specific download numbers, but the original Goat Simulator remains a top-30 paid app. The goat, it turns out, is a cash cow.

The Goat Simulator you’ll find on Amazon Underground is the same as its iOS and Android counterparts, with one key distinction. Instead of a five dollar cover charge, you can play it for free.

That might seem like an odd move for an app that had already seen remarkable success with the transitional paid model. And at first, Coffee Stain Studios CEO Anton Westbergh wouldn’t necessarily have disagreed.

“The gaming industry is changing so fast, you never know what’s going to be the next big thing,” says Westbergh. “Our strategy is basically to try new things as they pop up.”

The initial response, Westbergh says, wasn’t especially impressive. But an influx of Fire tablet owners around the holidays resulted in a “huge spike” in daily downloads and new users.

Like Amazon, Westbergh also declined to provide hard numbers. The developer does say, though, that Goat Simulator revenue on Underground has outperformed its Google Play version by as much as 30 percent. That’s especially impressive when you remember that it’s comparing a five dollar app to a model that pays Coffee Stain Studios $.002 per minute of engaged user time.

As for what’s driving Goat Simulator’s Underground success, Westbergh can think of a few factors. “The audience might be different on Amazon Underground,” he says. “We think there might be a lot more kids on Underground compared to on the premium platforms. We charge five bucks elsewhere, which might be too much when kids ask their parents if they can get a game.”

Another reason ambient goat play might have found success is the nature of the game itself. Underground rewards long sessions, which benefits an open experience that mostly comprises roaming around and bumping into things. That, and the simple fact that Goat Simulator has been a roaring success in every arena it has entered.

That’s also why not every app should expect to find the kind of Underground success Goat Simulator. In fact, certain types of games may struggle in the pay-per-minute structure. "One thing we see as a potential drawback would be that it isn’t that optimal for a short game," says Westbergh. "There might be some really high-quality experience that is designed to be played for a limited amount of time that’s still worth paying for. Amazon Underground currently doesn’t really make that work."

Some of the drawbacks of Amazon Underground that existed at launch for customers persist as well. The selection still feels like a collection of second-run hits, although titles like Monument Valley and Star Wars: Knights of the Old Republic have aged well. And there’s still that whole multi-stage download process for non-Fire device, which isn’t going to go away any time soon.

That’s partly, though, why this growth spurt is so encouraging. The more Underground proves itself as a viable path for developers, the more developers will risk putting newer material on Underground.

“Once Underground can prove that an average game can do at least as well as on other platforms, I think that’s the turning point,” says Westbergh. At this rate, it shouldn’t be long at all.