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...
Your comment reminded me of "No Time on Our Side," a book about a submersible laying cable near the UK that sunk after a hatch failed during recovery operations. The author (who was a pilot in the submersible) details the incredible rescue effort to bring them back to the surface alive over a period of about 3 days.
A wonderful book and also one that made me appreciate how hard it is to lay cable (in some places).