UBI generally has the implication attached to it that creating a proper safety net allows for savings in other areas, simplifies other social services, and reduces costs through net social benefit - not just the upfront costs.

So health care savings, the ability to quit your job and go back to school (or other kinds of re-training, generally a diversified workforce is a beneficial one), reducing the need for shelters, etc.

This is aside from the fact that a reduced engagement in the workforce doesn't have to be a negative thing - plenty of money is being made, it's just not being distributed very well :) UBI just intends to right that inequality in a productive way.