Can I use formulas in a browser grid?

No. CGXBrowserGrid has been designed to interact with external data sources or data bases that are “record oriented”. The user makes changes in the current record and once he navigates to another record the records data will be written to the data source all at once. CGXBrowserGrid holds no data in memory and only loads data on demand when the user scrolls through the grid. Because of this specific design of the browser grid it is not possible to support formulas.

You have to use a regular grid (not derive from CGXBrowserGrid) instead.