Great question - my firm builds and helps migrate/support dozens of mobile apps each year. There are several questions that need to be answered before a reliable estimate can be given. For example, are you strictly seeking a native mobile app? If so, which platforms? Will the mobile apps shared a backend with the existing web application, or will it be stand alone? have you outlined the UI and UX yet? produced wirefarmes, technical specifications documents or any information architecture outlines?

Simply put, there are items in most mobile app projects that do tend to require more time investment (which next to the location of where you build the app is of course the most important driver of cost/quality): this would include things like UI/UX, API documentation, Technical Documentation and Testing- most of the time, the actual core development of an application (assuming you are investing in the above appropriately) comes down to block and tackling and just executing the strategy you lay out ahead of time.

All of that being said, based on the limited information I have now, this certainly sounds like an application that could be on the faster side of the delivery timeline. If you'd like to get a few drilled-down and accurate tiers on pricing, timeline and some suggested next steps, feel free to reach out and we can discuss further over a quick call.