Lets do a little Math... A cable that moves 4GW of electricity 450 km is say +-1 million volts and 2000 amps.
Assuming we want no more than 3% losses in the cable at full load, then each conductor needs to be 40mm diameter aluminium, at a total material cost of $3.6M
To insulate a 1 million volt cable, we need 100mm of PVC - total cost $60M.
And we'll obviously need a few mm of steel + more PVC on the outside for protection from the environment.
And now add in the manufacturing cost, and the cost to get it into place...
They are normally operated together, with equal current going through both.
But it's also possible to operate just one, with the return current going through the earth. Personally, I think that's a very bad plan, because putting 2000 Amps through the earth will probably have lots of unknown ecosystem effects. Things like bacteria that navigate or hunt via electric fields will be killed over an area many thousands of kilometers. Also, the chemistry that happens in rocks and underground rivers is altered by electrical current flowing.
But maybe that ship has already sailed - nearly every country has some project that puts big electrical currents through the earth, so maybe we have already destroyed everything that cares, and therefore doing more of the same wouldn't be so bad.