Google Container Engine Costs for Small App

First post! Big fan of the the Indie Hackers project, Courtland.

On to my question: I'm building a small web app and considering running it on GKE. Curious if any of you have a little app running on the Google Container Engine and if so, what are you paying per month for that project with the load balancer and GCE instance costs. It sounds like I can get away with staying inside the free tier from the docs, but in practice there's always a hidden cost, right?