It'll be tough to determine a 'rough' cost given that are many variables to consider. For example, how old is the building, what type of mechanical system does it have, is there u/g parking, are there any common areas, if so, how large, etc.
To be honest, I would actually get a professional appraisal done in order to establish a figure that is produced by a professional appraiser. Particularly if this is going to be used for commercial insurance purposes. If you ever had to make a claim the insurance company could challenge the value that was applied.