Developers often drop a paid app to free for a day or two to climb the charts. There is no fixed schedule, so the trick is knowing where these price drops get reported first and claiming the app while it is at zero.
Google Play / individual developers
Paid Android apps and games regularly go free for a short time as developer promotions. If you 'buy' at $0 the app stays tied to your Google account.
Apple App Store / individual developers
iOS developers sometimes make paid apps free for a short promotion. Download during the free window and the app remains in your purchase history for re-download.
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