[QUESTION] What establishes digital currency value in a post-labor economy?

Many people are now talking about a Universal Basic Income. I can only infer that they are eluding to some form of mass digital currency which could be used to pay for the rooves over people's heads and the food in their bellies. There is the question then of what gives that currency its value, since it will no longer be compensation for labor, at least not human labor--this is the currency of a post-labor world in which robots and AIs do everything for us, and in which it has apparently been decided in advance that all citizens are entitled to all the basic of life. I have heard this referred to as "Star Trek Socialism." But even with AIs and robots doing all the work for use, something, some unit of currency has to represent value.

Of course, in business, the single largest cost is always human labor, so with that taken out of the equation, and the increasingly sophisticated, low cost ways of doing things, like 3D printing houses, the prices on the things we need should plummet. I'm just not sure I see where the incalculable of money that would be needed to pay for UBI would come from.