Hi Gerald and Michal,
Thanks Michal for referencing that document – take a look at this Gerald if you haven’t already as like Michal says it addressed this directly.
In summary – the budget is decided by you, based on what you think is reasonable for 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.). If the community is expected to pay for it, depending on what you are implementing, then you might look at the average income in that area using the Census and calculate what you think would be affordable for them to pay over the lifecycle of the project. Or, if it needs organisations or local governments or probono corporate involvement to deliver it, your budget can be higher and perhaps you can look for grants in the region for these types of projects to find estimated funds available.
Hope that helps – feel free to comment here again if you have further questions.
Best of luck with your project,