Ministers, including Home Secretary Alan Johnson, have said scrapping the scheme would save nothing because it would effectively be self-financing. But figures released by the Conservatives revealed the estimate was based on huge levels of take-up over ten years.
Shadow home secretary Chris Grayling said the prediction showed Mr Johnson was "utterly deluded".
Mr Grayling said: "If Alan Johnson seriously thinks that nearly half the adult population is going to voluntarily pay for ID cards out of their own pocket then he is completely deluded.
"It is time the government realised this whole scheme is a white elephant and it should be scrapped immediately."
The estimate is based on the ID card costing 30 to buy. At that price, 28 million card carriers would be needed to cover the cost of the project, which is estimated at 835 million.
However, the cost of the national identity register and new biometric passports brings the total bill for the scheme to more than 4.5 billion, according to the official Home Office estimate published yesterday.
That is a small fall of 365m on the last estimate linked to the contracts awarded to companies to issue the cards, a spokesman said.
Anyone who wants a passport or ID card will have to pay the expected 30 cost of having their fingerprints and other biometric data taken, in addition to the cost of the document itself.
ID cards will be available in Greater Manchester from later this year.
This summer, Mr Johnson ruled out making ID cards compulsory and ditched plans to require airport workers to hold them.