If your free school is approved, you will receive a Project Development Grant from the DfE in the pre-opening phase of the project - after the application has been approved, but before the school opens. This is to enable you to complete all the tasks in the run up to the school opening. The amount varies depending on the type of school you are proposing, and more details can be found in the DfE's pre-opening guide.
Free schools also receive a capital grant to cover the costs of procuring, building or renting a site as well as any refurbishment costs, and the implementation of ICT infrastructure. The amount of this will very much depend on the individual nature of the school and the location in which it is situated and therefore there is not a fixed amount for this.