I won't give you exact numbers to use because your definition of basic modules isn't clear. However I'm assuming you're the vendor of the software hence you're acting as a consultant to provide the said ERP to the school. Keep in mind that the school has a budget of their own which is against yours. So some components to guide you for costing your ERP to the school would be:
- licensing fees
- Additional servers and network hardware
- Data conversion and transfer to new ERP (If any)
- Customization (if necessary)
- Vendor/consultancy support post implementation
Maybe not all of the above would apply but ideally you get the picture of how to shape up your pricing model. Make room for contingencies though. Also, to advise you further; are you providing the said ERP as a cloud based SaaS solution or an on-premise solution with perpetual licensing?