This depends on how many tasks an author can take on themselves versus how many they can contract out (ie., oursource). In monetary terms, if an author is able to do all the tasks themselves, and avoids using distribution platforms, services, or marketing that require payment, then it could conceivably cost them nothing to self-publish. However, what doesn’t get spent in money, costs in time. For those authors using publishing service providers or the subsidy publishing model, it can cost from a few thousand to $10,000 or more for one title, depending on the number of services purchased. Here’s a breakdown from a publishing services provider, based in the United States. (The WGA provides links as general resources and does not endorse any third-party services or websites.)