Its interesting that you ask about cost per unit distance.
Wind is free, so distance is almost irrelevant. You can cross an ocean without using any fuel.
However, you must eat, cook, drink
water, be entertained. IMHO, most people think of cost per unit of time. How much $ per month for example.
Coming full circle back to distance, my
rule of thumb is 1000 miles a week, and 100 miles a day. That applies for passages. For coastal cruising, assume 5 knots and about 10 hours a day, so 50 miles is a good day.
These general rules of thumb used to truly annoy a
skipper friend of mine. 100 miles a day does not equal 1000 miles a week. 5 knots does not equal 100 miles per day. Also, it does not consider the size of boat,
hull speed, rig, etc. What truly annoyed him though, was that using that
rule, I was able to predict the time required for us to cross the
atlantic ocean, correct within a few hours. I believe he started the
engine to WOT on the last day just to be sure I wasn't exactly right...which would have killed him, figuratively.
I believe I got those numbers from a sailing book, but can't remember who said it.