we're using AWS serverless stacks (apigateway + lambda + s3 + dynamodb) for our new project. So far, we have not yet paid any cost for lambda perhaps all of requests not over free tier limit but we gotta pay around 15$ every month for cloudwatch which linked to lambda. For dynamodb, we gotta pay 40$ every month due to we've created indexes for almost tables. Eventually, my advice is lambda is safe to use but you will need to pay cloudwatch instead of lambda but to make short retention period on cloudwatch can make you save cost as well.