I have been using the nvt devices recently, which has been a game changer. 95 percent of elevators have a pair of 16 or 18 gauge wire available, so I normally do not even investigate this. Last one I did was 13 stories. Make sure you put in your quote that customer pays for elevator company time. Normally half a day with us on site and the elevator company we can do a single or dual elevator. The NVT sends POE over the 2 wire, so power is not even needed. Just run a network drop to the machine room.
I do not see a reason to use any other methods. The NVT device pair with 2wire balun is about 600 bucks. Usually quoted over the phone without site visit for 2k per elevator plus customer costs for their elevator company. Before this, the investigation often led to costly cabling adds to the travelerer cable, which many times led to customer balking at the final project.
Be sure to let the elevator company be there for installation (I have elevator conpany install or insist with installation of the camera, since the elevator and cabling is their liability).