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It costs about $3 million per mile to convert underground electric distribution lines from overhead, while the cost to build a mile of new overhead line is less than a third of that, at approximately $800,000 per mile, according to a section on PG&E's website called Facts About Undergrounding Power Lines.

...PG&E, the state's largest utility, maintains approximately 81,000 miles of overhead distribution lines and approximately 26,000 miles of underground distribution lines. It also has about 18,000 miles of larger transmission lines, the majority of which are overhead lines.

At a cost of $3 million per mile, undergrounding 81,000 miles of distribution lines would cost $243 billion. PG&E has 16 million customers; distributing that expense equally would amount to a bill of more than $15,000 per account.

USA Today source