If you're Curitiba or on the LA Orange Line, your BRT looks like light rail (done with buses).

If you're in most of the US, BRT is more like limited stop and express buses that get reserved lanes that are hard to enforce properly (anybody have a YT of riding a bus on Mission St. in SF?), special buses and paint jobs (possibly with bigger or more doors for faster on/off at stops), off-board fare collection (machines at the stops) in some cases, and signal priority of some kind. In one similarity to light rail (compared to streetcars), the running times are a bit quicker mainly because of fewer stops. Of course, you still need a local bus line on the same (or a very nearby parallel) street to collect distribute riders at places between the BRT stops.

BRT, in general, is less expensive than light rail, mainly because you don't have to build a lot of new stuff. You're mainly restriping and fixing up the streetscape of an existing road, and possibly buying new buses that are a bit nicer to ride that the garden variety. Unless of course it's the LA Orange Line or Curitiba.

BOB2 or Transportation Planner has the numbers. My impression is that BRT carries somewhere between the ridership of a standard limited-stop bus line and a poorly-used light rail line.

A key benefit of light rail, of course, is that it's not on the street (usually), and that it runs TRAINS - multiple passenger cars with one driver (reducing operating labor cost). Until/unless road vehicle automation reaches the point where one bus driver can control a closely-spaced train (which is not impossible, but not here yet in practice), BRT will not be able to match that crucial benefit of light rail.