This is such a subjective issue, it's hard to know what is "reasonable". To be sure, the perception is that the chains overprice their products. One of the more successful chain outlets around here does very well at the boxoffice, but not so much at the snack bar. They have something like 6 or 8 sales positions, but I've never seen more than 2 behind the counter.
I get $5 for a cheeseburger. Comparable sit-down places charge at least that much. At the same time, you can get a drive-through gut bomb for a buck or so. My pizza prices are about mid-range for the area. Sit-down pizza shops are somewhat higher... the drive-through "take 'n bakes" are lower.
I'm amazed at some of the prices I see at drive-ins in the central states... $2.50 cheeseburgers, $1.25 hotdogs, $1.50 popcorns & $.75 sodas. These same theatres constantly complain about being "on the edge". Maybe they are, but I see a lot of underselling at drive-ins, and I can't see any good reason for it.
Years ago, a fellow owner told me that the time to raise prices is when you're so full you can't make more money by improving the product. There may be some truth in that. Still, there's a perception that deeply discounted products are priced that way to compensate for problems elsewhere, so maybe you can go overboard both ways.
My feeling is that price is somewhat secondary to how you present the product... within reason. If the customer perceives they're getting an outstanding value, there's less reason to be concerned about the price. They very well might think you're the most expensive popcorn in town... but if they believe you have the best popcorn... for whatever reason... I'd bet you don't get a lot of complaint. I see this when I go to places like "Outback Steakhouse", or "Olive Garden". Neither of these restaurants could be called cheap, discount, or even inexpensive. You can get what they sell at lots of places for a lot less money. Still, both places provide a product and an atmosphere that I think is worth the OCCASIONAL splurge. It's not an everyday thing, and that makes it more special.
Everything I sell here can be bought somewhere else for less, though you'd have to go get it. You can also find everything I make somewhere for more. I'm happy with the balance, and the majority of my customers appear to be good with it, too.
For you? You'd have to decide what's reasonable for your area. If you're near New York City, you might get away with a $10 hamburger. In farm country, maybe not... but I'd bet your overhead would be lower there, too.