Here's a new question: what is the most expensive scheduled flight you know of on cost per mile basis? It's possible that there are many good answers in remote parts of the planet. The one I'm thinking about doesn't involve ACK or MVY, but it is a flight between two islands that are 20 miles apart. The runway at the second airport is only 2100 feet long, and the approach entails clearing a mountain pass, so only smallish airplanes are permitted. They require a special pilot certification in order to fly there.

I suppose a logical companion question would be "least expensive", but this isn't as fair because mistake fares used to be quite common (e.g. Bhat runs).