I just got a response on this question from a theater financing company. They don't know of any hard data available on it for the total market, but they gave some numbers for planning purposes anyway: regional differences from mid-$2 to high $3; places with cafes and/or pizza over $4; and a "meaningless" national average of probably $3.25 before taxes.
Per capita income in my target area is about 68% of the national average, and taking 68% of $3.25 gives me a figure of $2.21 per cap for concessions. That's certainly lower than I'd like to aim for, but on paper it looks workable for turning a profit as long as building and land costs don't get out of hand.
Do these numbers seem reasonable?
And one more note. Info on the publicly traded companies suggests they get in the neighborhood of $2.50 to $2.75 per cap, but from everything I can find, it looks like fewer screens tends to translate into higher per caps (I'm guessing due to shorter lines, better service, possibly lower ticket and concessions prices). Is that a reasonable assumption?