The cost of running a production server is something small startup businesses need to budget for... And you don't want to run your production website on a 5-10/month shared server. If you design for the cloud/serverless, you can get your costs down to near-$0 for light usage... But yes, using things like lambda mean a possible bill shock if your traffic grows faster than your income can recover it.. but if you have an advertisement on the page, it probably will cover the execution cost if it's lean enough.
In another example, I inherited a CICD pipeline that cost $300/month and could run 2 of our larger test suites simultaneously.
I converted this to AWS Spot instances and now support thousands in parallel and I pay under $170/month ( https://ldoughty.com/2018/01/gitlab-multi-runner-w/docker-ma... )... But to your point about "what if" costs of this, yes, there's a limit on place for this setup so it can't go over 20 or so... And it is important to consider the impact of auto scaling with regards to the bill... For a small business, AWS effectively doesn't have a way for you to really stop "Bill attacks" or legitimate high traffic on API Gateway/Lambda short of you taking down your site.
My recommendation is: ideally, don't expose something that costs you much too run unless you can recoup the cost from that action (paying customer, advertising).. your homepage should be static or cached in a CDN until you're ready for that large bill.