I worked on a small SaaS that deployed in this way. The AWS bill was about 10x what it would have been if they had just deployed nodejs on an EC2 instance.
The point of doing something like this "Lambda Monolith" is that transitioning to an always-on server is trivial. I've been doing this exact setup for a while and our static costs basically evaporated. Seeing your costs drop below a dollar/month on a project you are building is amazing. As soon as one of our projects is large enough, we move it to a dedicated instance or cluster. This allows us to delay that point until we have enough revenue to justify the costs.
If you have one or more professional developers working on a project how can you be bothered about an extra $20-$200/month when you know you'll also be spending a couple of engineering hours (worth more than $200) on migrating later?
If you’re in territory where running locally isn’t feasible you’ve either done something horribly wrong or have scaled way past the point where either a single lambda or single ec2 instance is sufficient.
> Seeing your costs drop below a dollar/month on a project you are building is amazing.
Honestly, this just means you don't have any users
Which is where many small ideas start.
My buddy had an idea for a few SaaS products and I've helped him develop them. The initial costs were small, but give it was an expense with no income in the near future, finding a way to scale to zero was the only way this became something he'd consider. It's got _very_ sporadic use currently and his bill for each service is literally around $1/month right now. Even the cheapest instance would be more than he's paying.
All system architecture is about designing a system to meet the defined needs. This system is designed to hit the starting needs and has an already designed migration to the medium-term needs. That idea of "YMMV" is very much relevant here as every system has different needs. Some of those needs are minorly different, others are wildly different.
Anyone on this thread that's discounting FaaS usage out of hand with statements like "just run a $20/month server" are demonstrating a lack of imagination. Would I use it as the primary server for a medium+ traffic site, almost certainly not. But there is a whole world of low-traffic services that are well suited to using a FaaS platform over a dedicated server.