In recent years, a wide variety of wireless access networks that support multimedia services have been emerging with different characteristics. The service areas of many of these networks overlap so that a mobile user from an overlapped service area can access any network that supports the user's application. A mobile user can take advantage of the availability of such heterogeneous multimedia networks only when the user terminal is equipped with a mechanism that can select an optimal network for the application. This paper proposes a novel analytical framework of such a mechanism. For this purpose we define the user-centric performance and user-centric cost, and derive their expressions. The user-centric performance, which is also a measure of the user-perceived quality of service (QoS), relates the requirements of user applications with the parameters of network services. The user-centric cost is measured by the user's willingness to pay the price and allow for the consumption of the mobile terminal's resources for accessing the network services. Based on the performance and cost analyses, we describe a network selection mechanism and discuss its implementation issues. We then present an example system to numerically elaborate the functioning of the proposed mechanism.