What would be a professional developer's rate then? Should one charge higher if the project involves building the back-end yourself? Also, exactly what does it

What would be a professional developer's rate then? Should one charge higher if the project involves building the back-end yourself? Also, exactly what does it take to qualify as a "professional" developer, by which I don't think you merely mean being a mobile developer full-time, but actually having some set of qualifications?