0.08$/install with facebook is very good. the question is how many people leave your app installed.
try to make your campaign more aggressive and get 1000 installs per week. this will confuse google play algorithm and put your app in the top of the list and that will bring you free installs. it is not cheap to market your app. i spent until today more then 1000$ on app marketing and still did not get the result i wanted (even not near to what i wanted)
what i used for marketing is: facebook, google adsense, youtube videos by youtubers with lot of subscribers, forums, friends/family, printed flyers and more.
sometimes i think it simpler to create an app then promote it ?