> wouldn't UBI tend towards an inflationary spiral?
A UBI higher than the economy can support would seem to drive inflation, especially if we're (literally or effectively) printing some of the money.
Whether that leads to a "spiral" depends on what we do in response. If our UBI is intent on delivering a certain real value, so we give an $X that we adjust regularly to account for observed or expected inflation, then we run risk of inflationary spiral. So let's not do that.
If our UBI is a fixed dollar value, increasing based only on targeted inflation, then inflation drives down the real value to something we can empirically support.
There are also proposals that tie income to a specific revenue stream (a carbon tax, or Alaska's permanent fund), which has its own dynamics but doesn't seem to risk inflationary spiral.
> But without price controls, what would prevent rent/mortage/food/entertainment prices from just rising to absorb everybody's newfound income?
Competition, of both direct and indirect substitutes; this is the same thing that prevents rent/mortgage/food/entertainment prices from rising to absorb everybody's oldfound income.
Considering rent and mortgage in particular, as they seem to be in more limited supply than the others, I want to note two things:
First, competition is not merely competition amongst like units. If there is a fixed number of rooms for rent in a city, more people that want rooms than rooms available, and everyone can pay more, we should expect prices to rise. But as prices rise, choosing to take on an additional roommate, keep living with your parents, or similar become more attractive. And of course, the number of rooms for rent is only approximately fixed, even in the short term. These will typically stop the price from rising the full amount.
Second, we aren't actually particularly limited on housing available; we're particularly limited on sufficiently desirable housing available. A huge portion of what makes housing desirable is that it is sufficiently close to sufficient sources of income. By reducing necessary income, we are actually directly increasing the supply of "sufficiently desirable" housing, even before anyone starts building.
> College tuition has been going up exponentially since the introduction of federal student aid.
There is a very different dynamic when we give people dollars that they can spend as they need, versus when we give people potential dollars they can only spend for a single purpose. If I save a dollar on tuition today, and I'm borrowing from a lender that will only lend to me for tuition, then I can't turn around and use that dollar for anything else that I need (or want). This means it's less important to me to save that dollar, so there's less pressure keeping the price down.