A NSW MP has pitched the idea of building a 50-kilometre tunnel through the Blue Mountains to tech billionaire Elon Musk, who quoted "a bargain" to build it, according to tech billionaire Mike Cannon-Brookes.

Former Greens MP Jeremy Buckingham, who will run as an independent at the next state election, tweeted to Mr Musk, "How much to build a 50km tunnel through the Blue Mountains and open up the west of our State?"

Elon Musk has costed an idea by a former Greens MP to build a tunnel through the Blue Mountains. Credit:Bloomberg

Mr Musk, the American billionaire entrepreneur and founder of the electric car company Tesla and The Boring Company, which is aiming to transform high-speed transportation through tunnels, replied saying it would cost around $750 million with additional costs.

He didn't specify if he was referring to US dollars, in which case it would make it $AUD1.045 billion.