The Federal Opposition's Communications spokesman has stopped short of saying whether he would support the National Broadband Network if a cost-benefit analysis backed the project.

Malcolm Turnbull and Federal Communications Minister Stephen Conroy have gone head-to-head on the National Broadband Network in a debate on ABC TV's Lateline.

They argued over the cost of the Federal Government's proposed network, which will see broadband fibre optic cables connected to Australian homes.

They also disagreed over how the program should be implemented.

The Opposition has criticised the Government for committing to the $43 billion project before doing the cost-benefit analysis.

But Mr Turnbull will not say if he would support the project if the analysis was positive.

He says too much taxpayer money is at risk.

"Hundreds of billions of dollars have been wasted by governments - and indeed by the private sector - on the basis on building infrastructure in the hope that they will come," he said.

"Build it and they will come is a recipe for losing tens of billions of dollars."

The Coalition says the cost of the project will be $4,000 per household.

But Mr Conroy says that is a misleading figure.

"This is investing in an asset that will last up to 40 years," he said.

"If you take even Malcolm's $4,000, and stretch that across 40 years, it's about 13 cents a day."

Meanwhile, the world's richest man Carlos Slim Helu, who owns Mexico's national telephone company, says he believes the cost of connecting the network may be too high.

Mr Slim says the high-speed broadband plan should also focus on wireless technology, not just optical fibre, to connect homes and workplaces.