Microsoft Azure has a very helpful service, called Azure Advisor. Think about it as your personal advice on Azure services. Azure Advisor has many advice and recommendations about operations, availability, performance, security and cost.
Following the Cost section of the Azure Advisor, one can save a lot of money. Some of the recommendations that you will see and it is very impressive is about VM sizes. It gets the performance metrics of the running VMs. If the VM uses fewer resources than the allocated you will see a recommendation to reduce the size of that VMs together with the actual yearly cost savings.
Another impressive recommendation is the cost-saving calculation of the reserved instances. You will get a yearly estimation about the savings of your VMs if you acquire Azure Reserved Instances.
The Azure Advisor can be accessed at the Azure Portal. It is one of the pinned options at the sidebar. Click the Advisor and after the recommendations update you will see the advice.
Clicking the Cost recommendation box, you will see the detailed recommendation. This says that you can save this amount of money by buying reserved instances.