If you have to pay for users, your app sucks.

What I mean is, your revenue model should be such that:
cost/install * installs/registrations * registrations/purchases < # purchases * revenue/purchase

That's a long-winded way of saying your revenue should be greater than your costs. The problem is, the cost to acquire users has become prohibitively expensive in the last 2 years. The cost to acquire users on Facebook has multiplied by 5-8x. So unless you're selling big ticket items, it doesn't make sense. If you're selling an app subscription or in-app purchases, you've pretty much got to go with organic growth.

So, subscribe wholeheartedly to the growth-hacking model. Ensure that your product has a high viral coefficient, meaning that the app experience gets better for you the more people you refer. Provide incentives to sign people up. Get people to share, and then get them to convert.

Once you've reached a solid growth velocity (double digit MoM), and your conversion rate is high, it'll be easy to get money to pour fuel onto the fire.