A study by the state’s economist says Gov. Mark Sanford’s tuition tax credit would cost the state half a billion dollars over five years, causing one sponsor of the bill to reconsider her support.

The Board of Economic Advisors’ study — the first official statement of how the bill would affect the state budget — contradicts supporters’ claims.

Supporters have said local school districts would save money if some students took advantage of the credit and left public schools.

The bill gives tax breaks to parents with taxable incomes of $75,000 or less to pay for home-school expenses or to transfer to a private school or to another public school. It also allows anyone to donate to scholarships what they would have paid in taxes.

The BEA study estimates school districts will save $30 million mostly because they will have to pay fewer teachers.

But the state also would reduce its payments to local schools by $31 million — leaving a shortfall for districts of $1.5 million.

Overall, the economic advisers said that because South Carolinians would pay less in income tax, the state would lose $200 million in fiscal year 2010-11, when all S.C. families who qualify would be able to take advantage. The first four years of the plan would total about $300 million.

That has Rep. Shirley Hinson, R-Berkeley, a co-sponsor of the bill, questioning her support.

“I can’t support the bill with that cost unless there’s an explanation,” said Hinson, adding she anticipates Sanford’s office will offer a rebuttal of the study.

Sanford spokesman Will Folks said the governor’s office was still digesting the BEA study, which was released Tuesday.

Sanford and the BEA’s chief economist, Bill Gillespie, have clashed on studies of other bills.

Folks said Sanford is inclined to believe a study paid for by the pro-tax credit group and conservative think tank, the S.C. Policy Council. That study, done by a Clemson University professor, found the bill would save the state almost $1 billion.

The BEA study estimates that few parents with taxable incomes less than $35,000 — who pay little in income tax — would take advantage of the tax credit.

Parents with taxable incomes between $50,000 and $75,000 would be the most likely to take the tax credit.

Bill supporters mostly criticized the BEA’s findings.

“It’s what we expected from the BEA, another superficial, inaccurate portrayal of numbers,” said policy council president Ed McMullen.

Randy Page, president of South Carolinians for Responsible Government, called the BEA report “flawed.”

Yet both said the study bolsters their positions.

Page and McMullen praised it for confirming that the state and local districts will save money on each child who takes the credit.

Both also said the BEA report underestimates the number of students who will leave public school for private schools. The Policy Council study says more than 100,000 students would migrate. The BEA estimates 77,253 students will take the tax credit by 2010-11, including 10,000 public school students.

Opponents had their own questions about the study, but were largely complimentary.

“It is proof of what we said all along,” state Education Superintendent Inez Tenenbaum said. “This will cost the state of South Carolina millions of dollars to implement.”

Yet Tenenbaum added that she believes the BEA underestimated the total impact on school funding and does not believe it will cost teachers’ jobs. Tenenbaum said she doesn’t think enough students will take the credit to require a reduction in teachers.

Also Tuesday, a group of AfriAfrican-American ministers announced they will begin soliciting contributions from businesses to help fund scholarships for lower-income students whose parents do not qualify for the tuition tax credit.

Clergy for Educational Options said former state Education Superintendent Barbara Nielsen and Charleston developer Thomas Ravenel will lead the effort.

On Thursday, another group of black ministers will gather at the State House to oppose the bill.

Reach Talhelm at (803) 771-8339 or jtalhelm@thestate.comReach Gould-Sheinin at (803) 771-8658 or asheinin@thestate.com