Cloud hosting services offer computing resources which can scale along with the needs of users. When access to data is limited by the network capacity this scalability also becomes limited. To investigate the impact of this limitation we focus on bags–of–tasks where task data is stored outside the cloud and has to be transferred across the network before task execution can commence. The existing bags–of–tasks estimation tools are not able to provide accurate estimates in such a case. We introduce a queuing–network inspired model which successfully models the limited network resources. Based on the Mean–Value Analysis of this model we derive an efficient procedure that results in an estimate of the makespan and the executions costs for a given configuration of cloud virtual machines. We compare the calculated Pareto set with measurements performed in a number of experiments for real–world bags–of–tasks and validate the proposed model and the accuracy of the estimated configurations.