Balancing cost and reliability is essential when deploying applications on AWS, with On-Demand and Spot Instances offering distinct pricing models. On-Demand Instances provide fixed pricing without long-term commitments, but Spot Instances offer significant discounts of up to 90%, although they risk being reclaimed by AWS with minimal notice. A case study illustrates that running a multi-replica application on Spot Instances can be more cost-effective than a single-replica on On-Demand instances, with two replicas over 30 days costing 44% less despite a potential 50% interruption rate. Multi-replica setups on Spot Instances enhance fault tolerance and reduce costs, and tools like Kubernetes can mitigate interruptions by rescheduling workloads. Automation solutions can further optimize the use of Spot Instances by managing workload shifts between Spot and On-Demand instances, maintaining reliability while cutting expenses. This strategy is applicable across various cloud providers, demonstrating that Spot Instances, coupled with advanced orchestration, can effectively lower cloud costs without compromising service availability.