One of the reasons for said estimate is to test it's efficiency, which is being shown to be working well in the places that have been receiving it. The original idea behind only 12k was a compromise between advocates and opponents, with plans to increase to above 25K within a few years if it was shown to work, which it is.
RE: Basic income is about efficiently eradicating poverty. So if Bernie Sanders is right then minimum wage should be about $15 USD per hour, or about $30,000 per year. A lot of basic income advocates state basic income should be about $12,000 per year.