Really? The free tier comes with 28 instance hours per day. That'd mean your app would have to serve hundreds of requests per second, meaning each request must take substantially less than 10 ms, on a 600 MHz, 128 MB RAM machine.
If your request do any work at all, I doubt you can handle them in <10ms on a 600 MHz CPU.
Correction, each request would need to have less than 10ms of CPU time - the instances support concurrency.
My web frontend, by design, does very little - any CPU heavy operations are done by other systems using the task queue. Writing it in golang has helped as well, wouldn't get that performance from python.
Write a simple Hello world example in golang and get it to do some mathematical calculations to simulate "work", I think you'll be surprised at how many requests a second you can squeeze out of a single instance.
But still, 10 ms really is not a lot on a 600 MHz machine. How long does your front end take to serve one request? How many qps do you serve from a single instance?
I have some go code with a trivial, completely unoptimized blog, rendering a couple of articles. Poking at appstats suggests I spend a bit more than 10 ms CPU time, App Engine reports ~30 ms CPU time.
That particular request is authenticated, writes a file to google storage, and then returns a response (with a few other things like logging etc.)