Southwest Airlines offers four flights per weekday from Cleveland, Ohio to Tucson, Arizona. If adding a fifth flight per weekday would cost $15,000 per flight, or $110 per available seat, calculate the incremental costs borne by Southwest following a decision to go ahead with a fifth flight per day for a minimal 60-flight trial period. What is the marginal cost? In this case, is incremental cost or marginal cost relevant for decision making purposes?