You’ll need to erect a structure and a perimeter fence, buy the equipments and raw materials (and borehole), delivery van, registration (NAFDAC and CAC) and salary for one year. The cost of all these are not fixed and vary with your desired standard. However, you will need above 15million to build a standard automated bottle water processing plant. If you want just sachet water alone, that is far far cheaper.
If you are really interested, you can call me on 08033873233 to buy a detailed proposal on building bottle water factory, sachet water factory and others. My company (Habeam Nig. Enterprises) can also help you with the project.