I don’t know why they don’t do this at $1K/mo for life for 30 people (and then study them qualitatively) instead of $250 one-time for 20,000. What is a one-time payment of $250 going to tell us about a UBI?

The biggest problem with UBI is political. Even if you manage to implement it you have no guarantee that the next administration will continue it. If you can't guarantee that your $1k/mo will really be for life then you can't take the risks on things like starting a small business because you might have to go back to work one day and if you have a large employment gap that's going to be tough.

In this case you would set up a trust and fund it with the $5M so legally the only thing that could happen with the money would be $1K/mo to the recipients for life. I just meant in terms of what to do with this $5M that might be more useful for studying UBI.

because it would cost 18 million.

(I was assuming you could get a 7% annual return on the $5M, which is $350K, or about $1K/mo times 30.)