Cognitive Radio Networks have emerged in the last years as a solution for two problems: spectrum underutilization and spectrum scarcity. In this context we consider a paid-sharing approach where secondary users (SUs) pay for spectrum utilization. We assume a preemptive system where primary users (PUs) have strict priority over SUs, when a PU arrives to the system and all the channels are busy, a SU will be deallocated. This affected SU will then be reimbursed, implying some cost for the PUs service provider. This paper bears on the analysis of the behavior of the system where the number of users is arbitrary large and an admission control policy over SUs is applied. We develop a computationally efficient way to find an accurate estimation of the optimal admission control boundary based on the fluid limit technique. Our results are validated through numerical examples.