This research proposes to study users’ perceptions and behaviours when a commercial site wants them to fill a form on line. We propose to substitute the trust/risk approach with a cost/benefit approach, through the concept of perceived value. This allows taking into account the dilemma often felt by the user when he has to take the decision to provide the required data: do I have more to lose or to gain to reveal this information? We thus propose to test here a model aiming at determining the propensity of the users to answer a request of personal data on line and to provide or not erroneous data. We establish that the privacy concerns and the perceived value of the exchange have an effect on the reaction to the request in terms of filling the form and giving or not erroneous data. We also show that the sensitivity of the data results in a particular behaviour.