App Engine costs just as much as you use. There are many ways to tune or limit
how much you pay but in the end it all comes down to your usage of the platform. You're saying your endpoints cost you too much? What are you doing with them and do you believe you are accruing costs even as they stay unused?
If you're on the Standard environment with automatic scaling your instances can scale down to zero while you're not using them, is it not what's happening?