[bluGill]

Buses require expensive drivers and so they are the most expensive form of transit. A small bus costs almost as much as a big on to run. A train costs more per trip, but a train typically has many more people on. In theory you could run a train for service where a small bus would work, but since nobody does that the fact that a train would cost a lot more for that service is irrelevant.

[semi-extrinsic]

Buses are far from the most expensive form of transit. High-speed ferryboats or helicopters probably claim that prize. I'm pretty sure airplanes are also more expensive, as are subways if you include the cost to build them in the first place, as well as cars if you include total cost of ownership.

[bluGill]

Cost is typically measured per passenger km or some such. Ferryboats can come out cheap if there are enough people on them. I'm not aware of anyone using helicopters for mass transit. Airplanes are very cheap for long distance service.