Costs for the "immoral and obscene" replacement for the Trident nuclear missile submarines have already reached almost £5 billion before Parliament has voted to give the project the final go-ahead, the SNP said.
Funding for the concept and assessment phases of the scheme to replace the ageing Vanguard class boats which carry the deterrent stands at £4.8 billion.
At the time of the strategic defence and security review in November it emerged the overall cost of Trident renewal could reach £40 billion.
The Ministry of Defence estimates that acquiring four new submarines to carry the Trident deterrent will cost £31 billion over the course of the 20-year procurement programme - compared to a previous estimate of £25 billion.
And a further £10 billion "contingency" has been set aside to meet additional unexpected cost increases, reflecting the complexity of the project, with the first submarine due to enter service in the early 2030s.
The SNP highlighted that part of the cost increase was an extra £600 million for the design phase, taking the total cost for that element to £3.9 billion, on top of the £905 million concept phase.
SNP defence spokesman Brendan O'Hara said: "People will be rightly shocked that before the Government has even bothered to put this to a vote in the House of Commons - they are just pushing ahead, spending billions and billions of taxpayers money on preparing for these obscene and redundant weapons.
"A billion here - a billion there - all based on the arrogant assumption that Parliament will do what the Tories want and at a time when austerity cuts are deep and unending.
"We know that Labour have bottled it on Trident - with Labour and its Scottish branch office at loggerheads, so once again the SNP are the only effective opposition at Westminster.
"This cost would be indefensible under any circumstances - but at a time of deep austerity and swingeing cuts - it is completely unacceptable. Trident is the UK's ultimate virility symbol - it allows David Cameron to wield what remains of imaginary influence - proving beyond any doubt it is not a military asset - it is a political one."
Mr Cameron has assured MPs that they will get the chance to vote on the renewal programme "at the appropriate moment".
Mr O'Hara stressed the SNP's decision to oppose the project at every turn.
He said: "Trident is an immoral obscene and redundant weapons system - the deadliest nuclear arsenal in history. It is also one that is already costing us a fortune - and that is before a decision has been made. The SNP will vote against Trident at every opportunity - whenever the Tories get round to it.''
An Ministry of Defence spokeswoman said: "The UK's independent nuclear deterrent is the ultimate guarantee of our nation's safety which is why the Government is committed to maintaining it.
"We estimate costs will amount to around 0.2% per year of government spending - a small price to pay to ensure the security of British citizens."