As I said, the actual economics around this are more complicated, but the paraphrase/summary is that if nobody has to work for $1000/mo and everybody gets $1000/mo then $1000/mo will be worth practically nothing, with staples & necessities rising to consume that.
If we exclude rent from consideration, and apply the economics of UBI to just food: the average price of 1 Calorie (of which you need ~2000/day) rises $0.016, or $32/day. For a baseline reference, I regularly make healthy meals at $1/plate. Congrats, you've just increased the cost of food to at least $35 per day - precisely because "everyone now has $1000/month, free".
If we then roughly combine that with housing (as primary costs): I figure a normal baseline poverty minimum of $10/day for living space & utilities plus $3/day food, but then you're adding $32/day available which those necessities will instantly absorb (supply-and-demand) ... ergo you've just increased poverty-level living costs by 3.5x!
Unintended consequence: giving everyone $1000/month increases the cost of a $1 hamburger to $3.50. Now the beggar with $0 has to find/panhandle close to four times as much to afford something barely considered a complete meal. This is not what you had in mind.
