When estimating cost for a project, we have to determine the scope. There are 2 main variables that are used to determine scope. The breadth is the amount of features and the depth is the detail, quality, and complexity of each of those features. So in order to determine a “rough cost” we need to determine the amount of features and the complexity required from each of those features.
Its kind of like asking the question, “what is the total rough cost of a car”. There is a very wide range of answers to that question and the answer depends on the detail, quality, and complexity desired. That is why we recommend starting all of our projects with a roadmapping session because these questions are nearly impossible to estimate properly without doing a quick dive into these questions. After the roadmapping session we will have a detailed backlog of features we are committing to build and an estimate to accompany it.