Promised reforms to the system, such as the abolition of the so-called "bedroom tax" and more support for carers, could cost £350 million, according to new analysis by Better Together.
Meanwhile, the cross-party campaign group said the IT costs for setting up a new benefits system in an independent Scotland had been estimated to be £400 million by the UK Government's Department for Work and Pensions.
The SNP has already pledged to halt the roll-out of the switch from the Disability Living Allowance (DLA) to the new Personal Independence Payment - a change which it says will leave an estimated 105,000 disabled people worse off by 2018.
Nationalists have also vowed to scrap the controversial housing benefit changes that have been branded the "bedroom tax" by opponents of the UK Government.
At the same time, the party proposes to increase the allowance paid to carers, to bring this up to the same level as jobseeker's allowance.
But Labour social justice spokeswoman Jackie Baillie insisted the SNP's claims on welfare "simply don't add up".
Scottish housing minister Margaret Burgess hit out at the Labour, saying it would "rather defend Tory welfare cuts than recognise the urgent need for Scotland to make our own decisions when it comes to welfare".
Ms Burgess said: "Scotland is an enormously wealthy country, but for far too many people it simply does not feel like it. With a Yes vote we can make Scotland's resources work for people in Scotland and make our country a fairer place to live.
"It is a sorry state of affairs when Jackie Baillie and the Labour party would rather defend Tory welfare cuts than recognise the urgent need for Scotland to make our own decisions when it comes to welfare. Why does Labour want to see people in Scotland pay the price of deeply damaging and unfair Westminster welfare cuts that have devastated Scotland's least well off households?"
Ms Baillie insisted: "The welfare state is one of the UK's proudest achievements. It was founded on the basis of need, not nationality, so that those who fall on hard times get the support they need.
"To make that founding principle a reality we need to be able to fund our welfare state. Independence puts that at risk. The experts at the impartial Institute for Fiscal Studies are clear that leaving the UK would mean an extra £6 billion of cuts.
"The SNP's welfare claims simply don't add up. Alex Salmond needs to tell people in Scotland how his welfare promises would be paid for when independence would bring about austerity-plus."
The Labour MSP added: "It is those in the greatest need who would suffer most from the cuts that independence would cause. The nationalists are trying to deceive some of the most vulnerable people in Scotland. This is a risk that we just don't need to take.
"We have a better vision for Scotland. It's one where there are more powers for Scotland guaranteed, including more powers over tax and welfare, without taking on all the risks of independence. We should say no thanks to that risk."