Once you factor in the greater ongoing capital costs (buses must be replaced frequently, rail cars don't), ideal BRT tends not to be much cheaper than light rail. More than that, the ridership that justifies spending that much on a "real" BRT system means your shiny new BRT system will be at capacity basically right away.
Of course you almost never get "real" BRT as it's always easy to cut costs until what you're left with is just a bus. Buses are great, but they usually aren't rapid transit...
