Congressional Budget Office Cost Estimate: H.R. 653: Federal Intern Protection Act of 2017 [open pdf - 16KB]
"H.R. 653 would amend federal law to provide that unpaid federal interns are protected from workplace harassment and discrimination. (Paid interns are already considered employees for those purposes.) Although the federal government prohibits discrimination and harassment in the workplace through laws, regulations, and agency policies, unpaid interns are not explicitly covered by those laws."
Congressional Budget Office: https://www.cbo.gov/