The people who make under 10k outnumber the people making good money by a factor of 3-5?

I'm not sure that's the right ratio to look at - someone making under 10k isn't going to pay taxes at all. The average income in the US is just over 50k - I would expect that's about the income where your UBI would be 1 for 1 - your taxes go up 10k and you get 10k from UBI.