Earlier quoted context omitted.
The price of basic necessities is limited by the cost of production, not some fundamental scarcity. We could produce as much food, clothing etc. as anyone could want without increasing the cost. (In fact, the cost should decrease slightly due to economies of scale.)
If UBI truly is basic income that is livable then labor costs will go up, and thus production costs go up.
If someone is currently on $25k and gets a $10k basic income, their employer can lower their salary to $20k and they'll still be getting $5k more than they were before. (This won't happen immediately because people will be reluctant to take pay cuts, but I think it will in the longer term.)
Exactly how much of this difference the employee and employer capture in the longer term is a concern, but I think giving people the option to quit without starving will go a long way towards empowering them to insist on fair pay. I also think a result would be an increase job quality, because I think a lot of people would take a pay cut to work somewhere less awful if they could afford it.
Of course that money comes from taxes, so there's still a cost somewhere, but I'm not as convinced progressive taxes on the wealthy will have quite as big an impact on the costs to produce basic goods.