Outside of some nominal administrative cost in cutting the checks it doesn't "cost" anything on net, it just moves money around. Of course, the above-median person paying $14k in taxes but only receiving $12k back has lost $2k on a personal level, and that's really the cost to be calculated -- the reduction in incentive caused by the tax.
But if you use the model where the UBI replaces existing social assistance programs, marginal tax rates wouldn't be that different than they are now, and the marginal rates on lower income people would be lower when you account for the marginal losses they currently suffer to phase outs of existing means-tested benefits.