This PDF from Wisconsin puts the cost at $280k-390k/mi depending on the voltage of the line. They put the cost of underground transmission lines at $1.5M-2M/mi

This PDF from Wisconsin puts the cost at $280k-390k/mi depending on the voltage of the line. They put the cost of underground transmission lines at $1.5M-2M/mi for the same voltages. https://psc.wi.gov/Documents/Brochures/Under%20Ground%20Transmission.pdf https://psc.wi.gov/Documents/Brochures/Under%20Ground%20Tran...