Opposition proposals for mandatory jail sentences for knife crime will cost up to £80million a year, according to official figures.
An extra 1,345 offenders would be jailed under measures planned by Labour and the Conservatives.
Labour want mandatory six-month sentences but figures obtained by the Liberal Democrats indicate this would cost the prison service £21million. Tory plans for two-year sentences would top £80million.
Liberal Democrat justice spokesman Robert Brown said: "Labour and Conservative proposals are pie in the sky. Scotland's jails are overcrowded as it is.
"If an extra 1,345 offenders were sent to prison, we'd need to build another Barlinnie just to house them all.
"All the evidence suggests most of them would come out of prison worse than they went in, and a more serious danger to the public."
Labour lodged amendments to the Criminal Justice and Licensing Bill at Holyrood which would lead to the introduction of their six-month plans.
Party leader Iain Gray met with campaigner John Muir, whose son Damian, 34, was killed in a knife attack in Greenock three years ago as part of a Labour drive to confront the problem.
But a Parliamentary answer from Community Safety Minister Fergus Ewing indicates that an extra 1,345 convicts would be jailed for the offences of having in a public place an article with a blade or point, possession of an offensive weapon or restriction of offensive weapons.
The prison service estimates that prisoners cost about £31,106 to keep in jail annually, meaning the Labour policy would cost £20,918,785 while the Tory plans would hit £83,675,140.
Mr Brown added: "Liberal Democrats think this money would be better spent keeping knives out of young people's hands and battling against gang culture.
"Mandatory sentences are unworkable, ineffective and completely unaffordable. Possession of a knife in public is already rightly a serious crime, for which people can expect to go to jail.
"The appropriate sentence in individual cases must be a matter for the courts, not politicians."
Labour accused the Liberal Democrats of "gross hypocrisy" on knife crime. The party said Mr Brown has previously been quoted as saying that carrying knives is "stupid" and "should normally lead to a prison sentence".
