Context. Cloud computing is getting more and more interest with every year. It is an approach that allows Internet based applications to work in distributed and virtualized cloud environment. It is characterized by on-demand resources and payper-use pricing. Software-as-a-Service (SaaS) is a software distribution paradigm in cloud computing and represents the highest, software layer in the cloud stack. Since most cloud services providers charge for the resource use it is important to create resource efficient applications. One of the way to achieve that is multi-tenant architecture of SaaS applications. It allows the application for efficient self-managing of the resources Objectives. In this study I investigate the influence of tenant-based resource allocation model on cost-effectiveness of SaaS systems. I try to find out weather that model can decrease the system's actual costs in commercial public cloud environment. Methods. I am implementing two authorial SaaS systems: first tenant-unaware and then using tenant-based resource allocation model. Then they are deployed into Amazon public cloud environment. Tests focused on measuring over- and underutilization are conducted in order to compare cost-effectiveness of the solutions. Public cloud provider's billing service is used as a final cost measure. Results. The tenant-based resource allocation model proved to decrease my system's running costs. It also reduced the system's resources underutilization. Similar research was done, but the model was tested in private cloud. In this work the systems were deployed into commercial public cloud. Conclusions. The tenant-based resource allocation model is one of the method to tackle under-optimal resource utilization. When compared to traditional resource scaling it can reduce the costs of running SaaS systems in cloud environments. The more tenant-oriented the SaaS systems are the more benefits that model can provide.