This paper investigates secrecy rate optimization for a multicasting network, where legitimate transmitter broadcasts the same information to multiple legitimate users in the presence of multiple eavesdroppers. In order to improve the achievable secrecy rates, private jammers are employed to cause interference to the eavesdroppers. However, these private jammers introduce the charges for their jamming services based on the amount of interference received at the eavesdroppers. This secrecy rate maximization problem is formulated into a Stackelberg game, where the private jammers and the legitimate transmitter are the leaders and the follower of the game, respectively. First, we consider the fixed interference price scenario, where a closed-form solution is derived for the optimal interference requirements at the eavesdroppers to maximize the revenue of the legitimate transmitter. Based on this solution, we then derive the Stackelberg equilibrium of the proposed game, at which both legitimate transmitter and the private jammers achieve their maximum revenues. To validate these theoretical derivations, simulation results are provided for fixed interference prices and Stackelberg game scenarios.