I have sometimes wondered why they don't offer a separate, uncomplicated fixed price (and bounded resource) version - basically like a Heroku - and provide a migration path to their core offerings.
AppEngine is priced very high and is definitely not a constant price. I'm using it right now, and it costs me $5/day to run an Elixir/Phoenix app for ~2000 active users. I'm leaving as soon as my free credit runs out.