Katherine Courtney, programme director of the Home Office ID card team, said it was impossible to disclose financial details when the scheme involved commercial relationships with suppliers.
"It would not be appropriate to publish it as we are moving towards a procurement process, as much as the supplier community would like us to," she said, speaking at a conference on ID cards organised by the Institute for Public Policy Research.
The programme has been criticised by MPs, business groups and technology experts who said the government has failed to present a convincing case for investing billions of pounds on one of the most ambitious public sector IT projects undertaken in the UK.
But Courtney said it was unlikely the business case would be published because of concerns over commercial confidentiality.
The Home Office had conducted a rigorous analysis of the business case for ID cards, comparing the costs with the benefits, before embarking on the project, she said.
Last month the Home Office announced plans to use iris, fingerprint and facial recognition to verify identity, after trials had raised questions over the ability of a single biometric to perform an accurate match.
The government would take all three biometrics at the registration stage to ensure it could make an accurate match against people already registered on the central database.
But a single biometric reading would be good enough to verify people's identity for day-to-day services, she said. And in some cases, it may not be necessary to take a biometric reading at all. Customers could simply present their ID card number.
Courtney denied that taking an ID card number would expose the public to greater dangers from identity theft. Anyone who falls victim to ID theft could use their biometric cards to prove who they really are, she said.