This isn't necessarily true. I helped build a business on top of AppEngine. We did $80m gross in our first year. Our bill went up to about $500 a month. Why so low? Because we designed the system to scale from the start. We became experts in how to make the system work for us. We knew what would end up costing us money, so we engineered it from the start so it wouldn't cost us money.

Here is the case study written after I left the company...

https://cloud.google.com/customers/gearlaunch