Guaranteed income is a form of stability, which changes the outlook of recipients far more radically than a one-time transfer. People are more likely to make plans that increase their productivity in the future, such as fixing vehicles, going back to school, or leaving under-employed situations. The price fixing you describe already happens as much as possible, that's what drives inflation. UBI lets people walk away from bad jobs and re-distributes agency so that nobody is trapped.

> Aren't there factors in his initial state that 500$ a month won't fix

On average, no. Most poor people are poor because their income is low, not because they are spendthrifts.

> You're a union representing factory workers, wont it be tempting to demand a 500$ raise for everyone ? And then all prices of the output of the factory would be shifted up in price to compensate ?

In a competitive industry typically the result is that the company posts lower profits for its shareholders, not that the widgets increase in price.

> If we look at what the american experiment did with their 1000$ cheque signed by Trump, didnt it do exactly that ?

Exactly what? Cause the average family to be $1000 poorer, putting them at break-even in the long run? That's not really true.

> Isnt it true that the value of 500$ today depends on how hard it is to get in the first place, and if it's much easier, it becomes much less valuable ?

Yes, the problem is that the money is too easy for the wealthy to get. I won't tell you exactly how often I receive $500 dividends in my brokerage account, but every time without fail I still think to myself "wow, free $500, I can't believe I received that by doing absolutely no work."