Great question, and one of the more challenging things to navigate through the EWB Challenge.
There is no set budget for the projects – rather it is up to you to determine what is a reasonable and appropriate budget for your design solution. You will have to determine this based on the value it delivers, and depending on who the stakeholders are who would need to fund that project (including who pays for any ongoing maintenance if required, etc.). You can justify a larger budget, for example, if you can demonstrate value for money such as high initial upfront costs but the solution lasts 10-20 years with minimal maintenance.
We have put together a document FAQs – Big Tricky Questions (found in our Resources) which helps to navigate some of the thinking about budgets and value that your design could provide, and how to justify this in your final report.
Let me know if you have any more questions!