[dragonwriter]

> If average incomes went up,

Average after-tax income doesn't go up under any sane UBI scheme; it's redistributing income (ideally, primarily capital income—gains, rents, and otherwise).

> how long would it take for those with monopolies on basic needs to raise the prices?

Unregulated monopolies on basic needs are a problem that needs addressed either through regulating the monopolies or breaking then up, independent of UBI.

That said, redistribution will result in relative inflation of prices of goods disproportionately demanded by lower income segments, even in a competitive economy. The increase in nominal prices should usually be less than proportional to the increase in income, barring hard supply constraints that aren't addressed by the reduced demand for higher-end goods.

[dllthomas]

> Average after-tax income doesn't go up under any sane UBI scheme

Median probably would. Mean probably breaks even, yeah, at least first-order.