The budget amount in a project will be the calculated based on service rate and time allocated of each task.
Budget = Task Time Allocated x Task Service Rate
Here is one way to add a project budget:
Click on 'Fields' under Task Section.
Select Service Rate, Budget and Time Allocated from the list (Do this if you have not selected these fields to be displayed on your Task section).
Input rate and the time allocated for each task.
Refresh your page to see updated Budget amount on Financials section.