I copied an app from the App Store. Their business model was free desktop application but $20 for the iOS client and I really wanted it so I made a 'clean-room' implementation with CloudKit and charged $0.99 for a one-off cross platform sale.
Bit brazen, but the idea isn't entirely unique.
Edit: P.s email me at adam@adamfallon.com if you would like a promo code to get the app for free :).