They are fast to setup and require zero care and feeding. That's the big "time to market" cost.

Beyond that, depends on your use case. I love using Lambda for webhooks that may be called anywhere from a few times a day to thousands of times per day. Once they get to the point of being "oh wow, this one lambda is expensive", you can clearly afford the budget to move it to a real server. But below that $20/mo mark (which is more than 5 million invocations of a small function) - you're golden.