Okay so, there are a bunch of different agencies in charge of different types of social services. If you have UBI, those are no longer required. The money is coming from those programs. You spend LESS because you don’t have a giant work force on the back end of all those services/agencies anymore.
Eg. current: 20 departments, 100 people working at each. Gives out 1 million dollars a year in social services.
UBI: 1 department. Far less than the total of above working for it. Gives out 1 million dollars a year in social services.
See? The numbers are fluff just for the sake of the example.