For many years, a large number of charter boats have operated out of Boston Harbor that take people out to fish. In this competitive market, the price of a full day's fishing is $15 per person. Boats hold up to 12 people and will operate on any given day as long as at least 6 people sign up. On average, over the entire season, 8 people a day go out on each boat. The cost of a trip is the same regardless of the number of people on board. In light of the information provided above, what can be said (numerically) about the variable cost of operating a fishing boat trip in Boston Harbor?