BRT is a terrible solution. It can barely be defended when used with trolleybuses on lines that need something that isn't quite light rail.>>422641>>422640
You're right in absolute numbers, BRT is the cheapest of all systems, although not in the long run, and it gives the worst investment to service ration (cost/benefit).
Compared to trolleybuses, the next cheapest option, buses will be more expensive to maintain, and will have to be replaced much more often, also (especially on hilly routes) fuel will be more expensive and acceleration is worse. The amount of passengers that correspond to a BRT line make trolleybuses more economical in the long run, despite the cost of installation and maintaining of the overhead wires. If the line has enough passengers, the same goes for light rail: the added efficiency justifies the additional cost, and the investment to service ratio is much better, and a light rail system is cheaper per passenger-km than a trolleybus RT system, which in turn is cheaper per passenger-km than a BRT system.