I'm not sure your question can be answered as you state it. Here in NC, a BBQ restaurant out in some small town can rake in the money, while a similar restaurant in or near Raleigh, an area with about a million people, might just limp along. It's about competition, wages, building costs, tastes, etc.
Just to clarify the tastes concept: when I talk about competition, I know of small towns that have maybe 10-12 restaurants, with half of them being primarily BBQ type. All do well because folks grew up on BBQ. Then in other parts of the state, places with just one or two BBQ restaurants(and good one's at that) don't get near the business even with a larger population. It's all about tastes, and that's a hard thing to predict.