Virtually every residential installation has 2 pairs of copper lines running to the property already so if pair bonding was needed the extra costs are just at the node level.
In the case of overhead wiring, the 2 pairs go from the house to the utility pole. One pair is connected and actually in use. In the event of line quality problems between the central office or node and the utility pole connection, the lineman tries to find a "spare pair" that actually works to use instead. Based on my experience in the past, spare pairs aren't always available.
Another example: I switched from Shaw to MTS Classic TV, back to Shaw because I wanted a PVR and then to MTS Ultimate TV. On one of the switches to MTS TV, an MTS lineman came and installed an entire new line all the way from my existing drop that I was using for MTS phone to the node, about a week before the install date, presumably because there weren't any existing pairs that could be used.