If the only money is UBI money then things start to get weird. If UBI coexists with regular income in moderation then it doesn't change much. Consider that about 1/3 Americans receive some form of government assistance. There's already a kind of fallback UBI distributed across SNAP + Medicare + Medicaid + Unemployment + Social Security + etc, and no one on those programs is clamoring for them to be shut down so that lentils become cheaper. Giving money to everyone does increase inflation (though you can play with the tax rate to offset that), but the important effect is it transfers purchasing power to net recipients. Basically: the economy wide money supply would at worst go up by a modest factor, the income of the poorest goes up by an absolute amount (or a massive factor if you want to view it that way), which is a huge benefit to them.