When I was growing up Denver Academy cost $22,000 a year. Kent Denver was more expensive and Denver Christian and Saint Mary's were around $15,000. I am certain that all of these private schools have gone up in price over the last ten years. The vouchers would amount to $1000 a year. Who is going to supplement the rest of tuition. 

Title I money is meant to help schools that face the challenge of educating a lot of poor students; making it portable means some federal money would also go to schools that are generally wealthy but enroll a handful of kids from poor families. But turning Title I into vouchers wouldn’t be enough on its own to start a private school exodus. Dividing $14 billion in federal funding among the 25 million students poor enough to count as “disadvantaged” yields a voucher of $580. Private school tuition costs, on average, nearly $11,000 per year.