The current £42.6 billion budget makes it more than 10 times the cost per kilometre of other major countries.
The price has been criticised by campaigners, who said it is 'abysmal value for money'.
HS2 is estimated to cost £78.5 million per kilometre – double that of Turkey, Japan, China, Germany and South Korea.
France and Spain have built high-speed lines for less than £10m per kilometre while Italy built its for £43.4m per kilometre.
The Channel Tunnel Link, also known as HS1, cost £51.3m per kilometre.
Joe Rukin, Stop HS2 Campaign Manager, said: "Whatever measure you look at, the HS2 project has an abysmal value for money and with the £42.6bn cost still being worked out on 2011 prices, it is sure to keep on spiralling.
"If it is built, HS2 would be the most expensive railway in the history of the world, surely sucking up the entire rail infrastructure budget for decades to come, and squeezing out more deserving, cheaper and cost-effective projects.
"The whole concept of HS2 came out of intense political lobbying from the construction industry who want to build it. Those same firms have since been hired to produce the designs and budgets, and this has made sure HS2 has been gold-plated right from the start. HS2 is just one big gravy train for advocates with massive vested interests."
The high price has been attributed because of the vast differences in geography, labour cost and land prices, as well as the amount of bureaucracy faced.
Britain also has a little experience in building high-speed rail with just 68 miles (109km) of track.
In addition to the 29,658 miles ( 47,730km) a further 56,816 miles (91,437km) is currently being planned, according to data provided by the International Union of Railways.
HS2 will be 351 miles (564km) long and link London to Birmingham and Manchester and Leeds.
It will be built through 45 miles of Staffordshire countryside and include a link to Stafford railway station.
China has by far the most high speed rail links in the world, with lines extending 6,028 miles (9,702km) and trains travelling up to 217 miles per hour.
Trains on HS2 will travel up to 225mph.
Ben Ruse, lead HS2 spokesman said: "Europe has had high speed rail for decades and the continental network now measures more than 4,000 miles with a further 5,000 miles planned. In the UK, we have had around 70 miles of high speed rail for less than 10 years.
"As such, the development of new design and construction techniques being used on the continent may be applicable here to see if we can build high speed rail projects faster and cheaper.
"HS2 chairman David Higgins acknowledged this as a challenge in his report 'Rebalancing Britain' published in 2014. Senior HS2 engineers have since visited projects across Europe and worldwide to ascertain what lessons can be applied in the UK."