4 By the numbers . Suppose you operate a parking lot. The marginal cost of an additional customer is only 50 cents, but the fi xed cost of the whole operation in the short run for space rent, maintenance, and a ticket taker is $1,000 per day. The average daily number of customers is 200. What is the appropriate price to charge? Does the answer differ if marginal cost is falling instead of rising at that level of demand?