Assuming the radius of the earth is 6378.1 Km (it varies), the distance traveled over the circumference of half the earth will be 20037.375179 Km, compared to 12756.2 Km traveling straight through the earth. The difference is 7280.60115 Km. So the best possible case is a time saving of 7280601.15 m / 299792458 m/s = 24.285 mS (what parent said).
Expanding on that a little:
Using more realistic values, the distance between New York City and London is 5567 Km, while the path through the earth is 5392 Km (see http://imgur.com/Th6jW). So this would save 175000 m / 299792458 m/s = 0.583 mS
Expanding on that a little:
Using more realistic values, the distance between New York City and London is 5567 Km, while the path through the earth is 5392 Km (see http://imgur.com/Th6jW). So this would save 175000 m / 299792458 m/s = 0.583 mS
On the other hand, Spain and New Zealand are pretty much directly opposite (http://www.freemaptools.com/tunnel-to-other-side-of-the-eart...) each other, and Europe to Japan is pretty close to being on directly opposite sides of the earth.
So London to New York might be cost effective at $25 million, assuming the same $300M/6mS (http://www.telegraph.co.uk/technology/news/8753784/The-300m-...) rate, while best case (Spain to New Zealand) might be worth $1.2 billion.
Had to brush up on my geometry: http://www.mathopenref.com/arclength.html http://en.wikipedia.org/wiki/Law_of_cosines