I would recommend everybody to read the article above when deciding to use AWS Lambda for your next project. The article compare the pricing cost of AWS Lambda against Fargate, EC2 and EC2 spot instances. The article is technology-agnostic and it focuses on spinning up containers/apps for CPU-intensive tasks. The outstanding result of this analysis is that AWS Lambda is 2.4 more costly than AWS Fargate, and 7.1 compared to EC2.
Things change a bit when using Lambdas for IO-bound tasks with a technology like Node.js. AWS Lambda is severely constrained by only processing one task at a time, while a Node.js server on Fargate or EC2 could serve up to several thousands of concurrent requests, depending on your data storage. I would be very very keen in seeing an in-depth analysis for this case. I think that the analysis would still hold and probably the difference is even more steep.