For data delivery systems such as video-on-demand service, an optimum design is presented to maximize the revenue of the system with priority classes. The willingness-to-pay (WTP) is introduced for a measure of utility (price), and the optimum design is discussed to maximize the revenue. For the system with two priority classes, the optimum condition is given in terms of the traffic load, waiting time for service and pricing for the priority and non-priority classes. In this paper, we use the WTP as the measure of pricing. And we would like to examine the optimum design to propose the optimum pricing methods for time-base and flat-base. In this paper, using the WTP, the utility (price) of the data delivery system has been quantified, and the optimum condition to maximize the revenue of the system has been analyzed. From the numerical examples, the optimum condition for the service grade (waiting time for service) and pricing for priority and non-priority classes has been discussed.