This paper investigates the problem of optimally replicating objects at the candidate proxies in content distribution network. In our model, each proxy in the set of candidates has a finite storage capacity for replicating objects and charges fee for use. The optimization problem is to find a set of proxies from candidates set for replicating objects at them such that the total access cost is minimized, subject to the constraints that the objects placed at a proxy should not exceed the storage capacity of the proxy and the total fees charged by the proxies should not exceed a pre-specified budget. We formulate this problem as a combinational optimization problem and show that this optimization problem is NP complete. We propose two heuristics and evaluate them by simulation. The simulation results show that these two heuristics could significantly reduce the access cost.