I suspect it makes economic sense only when your workload is relatively elastic, having a relatively low duty cycle. The ability to pay less when you aren't actually using any resources is likely of more economic benefit as it becomes more fine-grained. If you aren't in that position, other models of lease or ownership of computing resources probably merit consideration.
Well, to be honest I pay 40€/month for a baremetal server (i7 SkyLake, 64 GB RAM, 4TB HDD). It's a powerful machine running many services, including a few virtual machines. I consider whatever it does would qualify as low-duty. Now, I know that each month I'm paying 40€ for all this. The last time about I read about serverless was when someone directly discovered the money saving part is quite tricky: https://news.ycombinator.com/item?id=14982220.
