Lambdas can be dirt cheap if you use them well. But that's always what happens. I recently saw a report about contractor who used them for crawling websites. Their client saw a surprise $12k bill for Lambda, when a simple Scrapy crawler on a low-end instance would have cost them < $100/month for the same load.
Why? Because if you spin up a ton of Lambda invocations and have them sit around waiting for the network, you pay for each CPU to sit idle, rather than having just one CPU stay busy managing a bunch of async IO.
Why? Because if you spin up a ton of Lambda invocations and have them sit around waiting for the network, you pay for each CPU to sit idle, rather than having just one CPU stay busy managing a bunch of async IO.