Cost: The money isn't going into a black hole, it's being given to those who tend to spend their money more quickly and more locally (aka the poor). So I would expect it to increase the velocity of money rather than decreasing it, giving the economy a nice boost.

Necessity: Any sane UBI system is revenue neutral. So anybody with above average income is going to pay more in increased taxes than they receive in benefits. That's easier and fairer than a claw-back.