This paper addresses the cost-aware virtual cluster placement problem in software-defined cloudlet networks. The objective is to minimize both communication cost and hardware cost of input virtual clusters. First, we present the mathematical formulation of the problem and acquire the conditions under which the communication cost of input virtual clusters can be minimized. Next, we focus on the optimal placement of input virtual clusters to minimize the hardware cost under the constraint of minimal communication cost, which is a Strongly NP-Complete problem with no fully polynomial-time approximation scheme (FPTAS) solution unless P =NP. To this end, we design an asymptotic polynomial-time approximation scheme (APTAS), which guarantees the performance to be within $(1 + \in )OPT + 2$. To reduce the time complexity of the APTAS solution, another greedy algorithm is proposed with linearithmic time complexity. Evaluations results validate the efficiency of our algorithms.