Reliability is a fundamental requirement of wide-area backup protection (WABP) systems, and it is one of the most crucial performance indicators for such a WABP system. Two definitions are introduced: the reliability of WABP systems and the undetectable probability of a power line. They are determined by the specific placement of communication links (CLs) and phasor measurement units (PMUs). A simultaneous optimization model is formulated to minimize the construction cost of WABP systems by optimizing the partitioning and placement of PMUs, protection centers (PCs), and CLs. The model takes into account several constraints, including latency, regional balance, and system reliability. To reduce computational complexity, a cluster-based genetic algorithm is developed to determine the optimal solution. Finally, numerical simulations are conducted using IEEE test cases. Results demonstrate that the proposed method can minimize the construction costs of WABP systems while increasing their reliability.