John McDonnell apologised if he came across as arrogant after he refused to say how much it would cost in borrowing charges to fund Labour's infrastructure plans.
The Shadow Chancellor said he had not wanted to get drawn into an argument that would undermine the fundamental idea that now was the time for the UK to borrow to invest.
He had said on Thursday that it was "trite journalism" to ask how much extra would need to be spent on servicing debt under a Labour Government.
He told Peston on Sunday that he regretted if he had come off as dismissive but he stood by his point.
He said that debates about costs risked getting "dragged down into an argument" about details which detracted from the bigger picture of the investment needed to grow the economy.
"That investment, as you know, wisely invested in our infrastructure, tackling our productivity problem, growing our economy, pays for itself," he added.
Mr McDonnell was asked about an Institute for Fiscal Studies figure, which suggested the additional interest would be around £2 billion a year, minus any nationalisation projects.
"I can' predict it. I'm being straight with you here," said Mr McDonnell in reply.
"As you know, government borrowing through bonds etc at the moment, the rate isa nything between 0.4 and 1.8.
"I can't tell when we go into government what that rate will be.
"What I can say is that the rates are at historic lows, this is the time to borrow."
The Treasury watchdog the Office for Budget Responsibility this week suggested that the budget deficit will not be wiped out until 2031.
Chief Secretary to the Treasury Liz Truss said: "Labour can't say when they'd eliminate the deficit because with their eye-watering borrowing binge, they never would.
"And their shadow chancellor - the man who doesn't do numbers - again refused to come clean on how much that borrowing would cost taxpayers.
"Labour would saddle future generations with a mountain of debt, meaning higher taxes and fewer jobs. Once again, ordinary working people would pay the price."