Calculations of the costs of tuition tax credits should include estimates of the induced migration from public to private schools. The estimate by the President's advisors that his 1983 tax credit proposal would cost up to $800 million by 1986 neglected the migration factor. It is demonstrated that a migration from public to private schools of about from 1 to 1.4% of the present public school enrollment is all that is necessary to secure savings from migrants that just offset the loss in federal tax revenue from tax credits granted to incumbent private school families. Greater migration will generate net revenue gains to governments as a whole.