+1 to what Matt. said. Enterprise account would likely be the most cost-effective solution. Here's the documentation from Apple: https://developer.apple.com/library/ios/#featuredarticles/FA_Wireless_Enterprise_App_Distribution/Introduction/Introduction.html.

There's is another alternative out there. After all, Starbucks gives away thousands of promo codes to paid apps all the time. I've been a developer on the other side of those giveaways—meaning, Starbucks featured our app.

Hope this helps,
Austin