Hi Lauren,
I have been advertising my app on facebook.
In other words, people who are using facebook (or instagram) will see my video ad. When they click it, it takes them to the google play store.
As far as budget, with FB you can set your budget.
So, for example, right now, my budget is set to $1 a day (I'm poor, lol). But, one can increase the daily budget to put their ad in front of more people daily.
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That is not too bad - how much was your budget?