Incentives are often used to address the problem of declining survey response rates. However, while they can substantially increase response rates, they can also significantly heighten research costs. Furthermore, their impact on survey responses often remains uncontrolled. This is especially unclear in mixed-mode surveys of the general population where web surveys are increasing included as a cheaper component. In this talk we discuss the problem of using incentives in web surveys within mixed-mode designs for surveying the general population. The experiment was conducted on a sample of the general Slovene population. Participants were randomly assigned into one of experimental groups, manipulating two different combinations of modes (web/mail and web/telephone) and three different types of incentives. We first present the impact of incentives on response rates and compare errors using the Mean Square Error approach. Then, a detailed analysis of bias in response patterns is performed. Finally, cost aspects are taken into account and evaluated by a simulation of different initial sample sizes and their effect on the costs-errors optimization. The findings suggest that the use of incentives in mixed-mode surveys should be methodologically well elaborated from the aspect of response rates, survey errors and survey costs.