Apple's developer accounts are unified, so there’s no difference if you want to develop for macOS or iOS or tvOS or watchOS. One fee gets you code signing for all and access to the paid stores if you wish.
Before the unification, If you were an iOS developer, nothing that you did on the iOS side changed when you subsequently enrolled for Mac development.
That being said, everyone can start with free Xcode and make an app for the shipping OS using the free SDK. You don't need to pay for a developer account until you need to sign code for other people or access pre-release SDK. Pay the money only when you hit a roadblock. Same goes for iOS, start free and only pay when you need to do something that the paid account allows. You can even run self signed code on iOS with a personal AppleID for personal testing before you become a paid developer.