Mr Slim, who is the world's richest man, was speaking in Sydney at the Forbes global CEO conference.
He said the estimate of $7,000 per home for the NBN fibre connections was too much. Because technology was changing constantly, he questioned why the NBN was focused so much on optical fibre, suggesting multi-platform network made more sense.
"Because technology is changing every time, I think to connect - I don't know how many houses we're talking here but maybe six million houses, seven million houses - $7,000 per house is too much," Mr Slim said.
While fibre was best used for backhaul connections, he said it made sense to use wireless technology for connections to premises.
"You need to have a multi-platform of everything: mobile, landline, fibre, cable and copper," Mr Slim said.
"I think fibre is not enough - you need to have a good network of wireless that you can connect it with in thousands of places with a wi-fi service."
Mr Slim's is worth $US53.5 billion (about $55 billion) according to the Forbes magazine rich-list. Much of his fortune was made through the Mexican telecommunications companies Telmex (the privatised national carrier), but he has also significant stakes in property development.