Can you explain in simple words what does it means?
I can see in many sites that they recommend using lambda for saving , and now you say that when you get lucky and grow the bill will explode?
Keeping a server running costs you X/hour. Having a lambda costs you Y/invocation. Which is cheaper depends on the number of invocations per hour. If you have 1 invocation per hour, a lambda will be cheaper. If you have 2 million per hour, a server might be cheaper.