The ground-breaking Channel Tunnel linking Britain and France was cheaper to build on a per-kilometre basis than Sydney's planned $16.8 billion WestConnex motorway, according to a report commissioned by a council opposed to the project.

The consultant's report for Leichhardt City Council also claims there is a higher probability than for most other large transport projects that the final cost of the 33-kilometre motorway will vary from its most recent estimate of $16.8 billion.

"At $16.8 billion, WestConnex would be considerably more expensive than other international and national projects on a per-kilometre basis," the report by SGS Economics & Planning says.

Whereas Australia's largest motorway project will cost about $510 million for each kilometre, the so-called Chunnel cost $426 million a kilometre and Sydney's Eastern Distributor motorway $223 million (in 2015 dollars).