In this paper, we present an economic cost analysis, including the capacity analysis subject to deployment cost constraint in the relay-enhanced cellular networks. Based on stochastic geometry analysis of throughput performance, we provide a framework to determine the maximum areal capacity with the given deployment cost, which involves the optimum RS and BS densities. Based upon the throughput analysis, a linear capacity-scaling property of relay system has been shown. Furthermore, we justify the advantage of low-cost relay nodes by deriving the capacity-scaling property that is governed by the economies of scale. Furthermore, our framework allows for analyzing the explicit benefit of relay system over the conventional cellular system as a unit cost of relay station varies. In particular, our asymptotic analysis has shown that the maximum possible gain is limited to 2.12 times as much as the maximum areal capacity of the conventional cellular system, no matter how many relay stations are employed.