Resource usage of google Go vs Python and Java on Appengine
Will google Go use less resources than Python and Java on Appengine?
  Are the instance startup times for go faster than Java's and Python's
  startup times?

Yes, Go instances have a lower memory than Python and Java (< 10 MB).
Yes, Go instances start faster than Java and Python equivalent because the runtime only needs to read a single executable file for starting an application.
Also even if being atm single threaded, Go instances handle incoming request concurrently using goroutines, meaning that if  1 goroutine is waiting for I/O another one can process an incoming request.

Is the go program uploaded as binaries or source code and if it is
  uploaded as source code is it then compiled once or at each instance
  startup?

Go program is uploaded as source code and compiled (once) to a binary when deploying a new version of your application using the SDK.

In other words: Will I benefit from using Go in app engine from a cost
  perspective?

The Go runtime has definitely an edge when it comes to performance / price ratio, however it doesn't affect the pricing of other API quotas as described by Peter answer.
Will google Go use less resources than Python and Java on Appengine? Are the instance startup times for go faster than Java's and Python's startup times? 
Is the go program uploaded as binaries or source code and if it is uploaded as source code is it then compiled once or at each instance startup?
In other words: Will I benefit from using Go in app engine from a cost perspective? (only taking to account the cost of the appengine resources not development time)