Sounds like a nice plan, except it lacks choice.
You can put your money into a hat if you want. Just don't force other people to, by taxing them.
We may never know the cost because we can never see what will now not be able to be produced. The higher income earners are the ones that produce many of the goods you take for granted that will be available to buy with UBI payments. It those entrepreneurs who arrange capital goods and labor in ways that result in better and cheaper products for the consumer. And they are rewarded for their efforts with profit. If you hamper their ability to do so. We will never know the true cost of UBI, which is all the goods and services which will not be created as a result of it.