Mr Goff was able only to promise his finance spokesman David Cunliffe would explain all in a spreadsheet at a later date - but many in the audience began to show amusement at his inability to provide more than that.
Mr Goff was able to claw back some momentum on the issue of National's partial asset-sale plan and again drew cheers when he said "nobody believes it's sensible to sell assets".
The debate was briefly disrupted as a man who had been heckling Mr Key was removed from the hall, but Mr Key easily regained the front foot on the cost of Labour's policies.
Picking up on Labour's pledge to follow National's plan to return the Crown's books to surplus by 2014/2015, Mr Key again asked where that $14 billion was going to come from.
"The reason you're not telling us is it's not there," he said.
The debate began on the issue of Christchurch's earthquakes and Mr Goff continued his strategy of injecting a personal note into attacks on Mr Key before 600 people at Christ's College in the Prime Minister's home town.
He jabbed at Mr Key and his Earthquake Recovery Minister, Gerry Brownlee, for going back on promises to quake-displaced Red Zone residents that they would not lose out under the Government buyout package.
"If you're not going to do something for heavens' sake don't promise to."
Mr Key defended the offer as "not absolutely perfect" but still the only one of its type in the world. If the Government hadn't taken that course, the badly affected eastern suburbs would have lost equity anyway.
Compensating affected homeowners was a balancing act. His Government had committed $5.5 billion to the recovery in the last Budget while holding spending in other areas.
His Government strove to be absolutely fair and reasonable, "but we've also got to be fair and reasonable to every other taxpayer in New Zealand because ultimately that's where the money comes from".
Mr Goff's personalised attack extended to the issue of insurance companies' reluctance to re-enter the Christchurch market which was hampering the recovery.
He wanted the Government to become the insurer of last resort if those companies don't return while Mr Key said that was happening already.
But Mr Goff said that was not happening fast enough and the risk was people would abandon the city.
Mr Key countered that such an action risked more damaging consequences further down the track. "I'm not going to rush in and do reckless things that would hurt Cantabrians."