For example, you need to develop a mobile messenger application for iPhone. And it is similar to the mobile application of corporate chat, which you take as a basis. The functionality is similar to the one you need. The interface (appearance) can also be similar.
However, your messenger will be used publicly by millions of users, which means you need to create a complex server part. Corporate chat (which you took as a basis) is only used within the organization, which creates another difference - user logic. Because of the publicity of your project, you will need completely different user scenarios, which leads to a more complex interface (design).
So while applications are similar in appearance and functionality, each project will be fundamentally different from the other. There will be a very different amount of material design, different user scenarios at prototype level, different set of software tools, different amount of code and other differences.
In order to name the cost and terms of your project, you need to discuss all the details with you personally and make up a detailed technical task.