Lambda architecture has gained high relevance for big data analytics by offering mixed and coordinated data processing: real time processing for fast data streams and batch processing for large workloads with high latency. However, concrete implementations over cloud infrastructures and cost comparisons are still not being sufficiently analyzed. This paper presents a cost comparison of Lambda architecture implementations using Software as a Service (SaaS) to support IT decision makers when streaming-analytics solutions must be implemented. To do that, a case study of transportation analytics is developed on three public cloud providers: Google Cloud Platform, Microsoft Azure, and Amazon Web Services Cloud. The evaluation is carried out by comparing deployment, configuration, development, and performance costs in a public-transportation delay-monitoring case study assessing various concurrency scenarios.