The present global poverty line as defined by the World Bank is $1.90 per day. I think a worldwide basic income of ~$57 per month ($1.90 x 30) would be the best way of eliminating absolute poverty for everyone on Earth, but that wouldn't be even close to sufficient for covering basic needs in OECD nations like the US where the poverty line is $12,000 per year.

$50 per month can be absolutely transformative for someone living in Kenya, but not for someone living in the US, though it would certainly be better than $0.

Basically, if the world can come together to do global UBI, great! But each nation will still need to implement its own basic income, and as mentioned in this post, that does not preclude states and cities from implementing their own top-ups as well.

By the way, the fact that poverty lines differ so much worldwide makes cryptocurrency solutions all that more powerful. For example, if Steemit paid out a Steemit dividend such that say 10% of the value of all posts went to every active account on Steemit as their share of the value of the network that is Steemit, that could be seen as pocket change by someone in the US, but the difference between eating and going hungry by someone in East Africa.