Paid CPI Advertisemen is just one kind of promotion methods. You can try many other methods no matter free or paid ones.
ASO is the best method to increase app install in a shorter period. What is ASO?
About app campaign, there are free methods also paid ones. Anyway, the most important thing is to get Organic traffic from real users all over the world.
1. ASO --This method can help you on app ranking with targeted keywords related to your App. ASOtop1 is offering this service to help it on keyword search&install from real users. When users do search the keywords you choose, they will find your app. With this method, your app will rank top in App Store search results.
2. Gain positive reviews-- BestReviewApp is one site to offer organic app install, play, rating and positive reviews from real users all over the world. This method will help a lot for app overall ranking.
SEO Landing page
You may create an landing page for your app. Then you will get some keywords related to it with SEO campaigns.
Post the download link, website, landing page address on Facebook, Twitter, Linkedin, Instagram, etc.
Press Release will obtain very positive results thanks to the “newness” factor associated with Apps. You will post your app features on the press release sites, related column, etc.
Once you published your app in Store, you can market it with cross promotion with other apps by installs and reviews exchange. It is free while it is still hard to find that publishers like you. You may try some Cross-promotion networks (CPN), Facebook group or exchange forums.
Make a video
Videos shows great ability even just 30 seconds, it has power to show users why the App is worth a try. Make up a story of your app? Show the real life? Yeah, both OK.
Make connect with your friends and families. You may ask them to help you spread your app in their relationship circle. Also, participate in meetings or conferences that are relevant to your App, and make full use of it to build partnerships.