Cost control for the Internet access providers (AP) influences not only the nominal speeds offered to the customers, but also other, more controversial, policies related to traffic shaping and discrimination. Given that the cost for the AP is determined by the peak-hour traffic (e.g. through the 95th-percentile), the individual user contribution towards the aggregate cost is not a linear function of its byte usage. In this paper we propose a metric for evaluating the contribution each individual user has on the peak demand, that is based on Shapley value, a well known game-theoretic concept. Given the computational complexity of calculating the Shapley value, we use a Monte Carlo method for approximating it with reasonable accuracy. We employ our methodology to study a dataset that logs per-subscriber temporal usage patterns over one month period for 10K broadband subscribers of a European AP and report observed results.