Scottish Labour yesterday cited figures from the Scottish Parliament's politically impartial Information Centre showing the plans outlined in the SNP's independence blueprint would cost £1.2 billion per year by 2024.
Leader Johann Lamont warned that even on the most optimistic estimates for boosting tax revenues the policy would not pay for itself - and accused the First Minister of trying to "con" voters.
Mr Salmond dismissed the claims, saying the childcare policy could be afforded in an independent Scotland just as it was in Sweden.
The row, which dominated First Minister's Questions, followed the SNP's pledge of a "transform-ational" expansion of free child-care if Scots vote for independence in next year's referendum.
The Scottish Government's White Paper, published last month, promises to provide all three- and four-year-olds, and vulnerable two-year-olds, with 1140 hours of childcare per year - the same amount of time children spend in primary school - by 2020, at a cost of £700 million per year.
By the end of 2024, the blueprint says, the same level of provision will be extended to all children from the age of one to when they start school.
Implementing the full policy would require new facilities and create 35,000 extra childcare jobs.
The White Paper does not give the cost of the fully implemented policy - but insists it would pay for itself through "economic growth and tax revenues" achieved by bringing 100,000 women into the workforce. The government estimates it would raise an extra £700m in tax if 100,000 women entered work.
Yesterday Labour argued that even if an extra 125,000 people took jobs thanks to improved childcare they would have earn £42,000 on average to generate the additional tax required to pay for the £1.2bn policy in full.
Addressing MSPs at First Minister's Questions, Ms Lamont said: "Will the First Minister now publish full costings of his flagship policy or admit it is a shameless attempt to con the people to Scotland and we've all seen through it?"
Later she called on the Scottish Government to use additional funds made available in George Osborne's autumn statement to improve childcare immediately.
She said: "If childcare is the SNP's top priority then they can prove it by delivering their promise of more childcare for two- year-olds now."
She accused the First Minister of trying to "change women's votes, not change their lives".
The call was backed by Scots Lib Dem leader Willie Rennie, who said the Scottish Government had a "golden opportunity" to match childcare provision south of the Border.
The First Minister told MSPs: "If you have more people in employment, spending more, it doesn't just mean they pay more tax. The people who benefit from the economic expansion also contribute more tax.
"It's the benefits of an economic expansion, as opposed to the austerity policies we have been suffering from Westminster over recent years from both Labour and Tory governments." He added: "That transformational policy in childcare can be afforded, can be pursued in an independent Scotland, just as it is pursued at the moment in an independent Sweden."
A spokesman for the Scottish Government said: "Improving female participation in work to Swedish levels would increase Scotland's economic output by an estimated £2.2bn and increase tax revenues across the range of all taxation by around £700m.
"Under devolution that additional £700m in tax revenues - and any savings from welfare payments - would go straight back to Westminster."