from an Edison Co. report:

"VII. Conclusion
Placing existing power lines underground is expensive. Undergrounding an entire state's existing overhead
power lines could cost as much as $94 billion and take decades to complete. The average cost of
undergrounding existing overhead power lines is approximately $1 million per mile. This is almost 10 times
the cost of a new overhead power line.
While communities and individuals continue to push for undergrounding—particularly after extended power
outages caused by major storms—the reliability benefits that would result are uncertain, and there appears to
be little economic justification or customer support for paying the required premiums.
Indeed, in its study of the undergrounding issue, the Maryland Public Service Commission concluded, "If a
10-percent return is imputed to the great amounts of capital freed up by building overhead instead of
underground line, the earnings alone will pay for substantial ongoing overhead maintenance," implying that utilities could have more resources available to them to perform maintenance and improve reliability on
overhead lines if they invested less in new underground facilities.37
For the foreseeable future, however, it appears that the undergrounding of existing overhead power lines will
continue, justified primarily by aesthetic considerations—not reliability or economic benefits. Many
consumers simply want their power lines placed underground, regardless of the costs. The challenge for
decision makers is determining who will pay for these projects and who will benefit from them."