Note the use of the past-tense "cost," as installation of two-wire copper local loops for telephony was done primarily during the mid 20th century.
In the first world there is no new infrastructure planning for new copper-based technology, and as customers are migrating to cellular telephony and high-speed Internet, wireline carriers are abandoning their copper local loops, tearing out the copper and replacing it with fiber-optic cable and/or selling the rights-of-way to third parties for private use.
Two-wire circuits in new installations are limited to intercom and military field telephone applications, though these too are being supplanted by modern digital communication modes.
A modern line card has no two-to-four wire conversion whatsoever; it is strictly an analog/digital interface to a system that has a completely digital and integrated signal path internally.
The old telephone hybrids of yore have been replaced by inexpensive IC chip-based components that perform the same functions at greatly reduced cost.