There's still a cost.
If you're a cost-conscious startup, you could use AWS Local Thingie to run it on a 5€/month VPS while you validate your idea.
Then if you get actual customers you could move it to proper AWS - or you might find out that Fake-AWS is actually enough for you and a 20€ VPS is enough for your uses and pay 0€ to Amazon, while still filing bug tickets to AWS Local Thingie Github for bugs and feature requests =)