I used my retired Android phone for the dedicated drone flight operations. And, it works very well.
The cost is 0.
And, here is the requirements of the Skydio App for your reference:
iOS
Check the App Store for compatibility with your specific device.
Device must be running 64-bit iOS 12.0 (or higher)
iPhone 6 (or higher), iPad Air (or higher), iPad Mini 2 (or higher)
NOTE: We're investigating an incompatibility between iPad Air (4th gen) and Skydio Controller and will update this list ASAP.
Minimum space on device: ~500mb
Android
Check Google Play for compatibility with your specific device.
Device must be running Android 9.0 (or higher)
NOTE: We're investigating an incompatibility between Samsung Galaxy 8 and Skydio Controller and will update this list ASAP.
Minimum space on device: ~500mb