Once again, opinions are a dime a dozen. Using the three levels noted above I'd say the three categories are "train set," "operating," and "collector."
"Train set" cars are made by Life-Like, Bachmann, Industrial Rails, and probably other companies that've never crossed my radar. They sell for less than ten bucks each and will most definitely need work to fit in on a model railroad. For kids, however, they're great.
"Operating" cars come from a variety of sources including Atlas, Athearn/MDC, Red Caboose, InterMountain, and a great many other companies I don't have space to mention. They used to start at under ten dollars, but unfortunately it seems only the Atlas "Trainman" line is less than that anymore. Generally, though, they run from $10 and up depending on the level of detail included. They generally come ready-to-run and with a variety of road numbers available for each road name.
"Collector" cars are a slippery bunch to nail down, though most consider them to be products from Micro-Trains. They're generally the best detailed and most accurate cars available and their cost reflects this. I, however, have no problem buying a Micro-Trains car, weathering it, and operating it on my layout. I guess it just depends on where your priorities lie. I'd also include what few high-end kits remain in the group as well.
~BS