Everything depends on your ambitions and plans. In general, as a service-based business, a digital marketing agency requires less amount of initial cost to start. However, you still have to hire people (at least, a developer, content writer, SEO specialist, graphic designer, and SMM) and let them sit somewhere if you are not working with them remotely. At a rough estimation, you may need $6000-$10000 to give it a nice start.