The "commensurate rise in prices" is the tricky part. I think what you would see would be a percentage increase in prices corresponding to the average percenta

The "commensurate rise in prices" is the tricky part. I think what you would see would be a percentage increase in prices corresponding to the average percentage increase in income. Since UBI is a flat amount, not tied to income, that average percentage is going to be so close to 0 as to be completely irrelevant. Even if it's not close to 0, for low-income (or UBI-only income) households especially, the marginal increase in income should be significantly higher than the marginal increase in prices.