You cannot. I asked Google about this, here's their response, from May 7 2016:
(GCE = Google cloud engine. No spending limits.
GAE = Google app engine — yes it has spending limits.)
... you are eligible for support on ... only ...
... [various helpful links] ...
That been said, at the moment there is no a feature that allows you to
configure a limited budget on GCE. This feature is certainly available
for GAE [1]. As you mentioned in your comments, you either can totally
shut down your VMs (will depend on your use case) or set the VMs to
send you alerts if they reach a certain traffic limit [2].
Sincerely,
Someone's first name
Technical Solutions Representative
Google Cloud Platform
[1] https://cloud.google.com/appengine/docs/quotas
[2] https://cloud.google.com/monitoring/support/notification-options
@wmdry, you wrote: "traffic to this link could explode" — I'm afraid of this too. That's why I asked Google about this. And I'm planning to avoid Google's CDN because of this, and use another CDN provider instead, which has spending limits. Because, unlike Nginx, I don't see any way for me to rate limit / throttle Google's CDN.
I do plan to use GCE (Google Cloud Engine) though. Therefore, right now I'm reading about how to rate limit my Nginx server. Because if I just configure Nginx correctly, then those $0.12 / GB you mentioned, cannot possible explode to ... like $10k in a month? What if Google sends a $10k bill when I'm back from an a few week's vacation, just because of my hobby project and a few people downloading a 1 MB movie over and over again forever (because: evil). Hmm, & the bigger & faster my servers, the higher the risk.
I hope Google will add spending limits, because I did want to use Google's CDN.
Update 2020: Apparently this does bite people from time to time — look here:
"Burnt $72k testing Firebase and Cloud Run and almost went bankrupt", Dec 08, 2020, https://news.ycombinator.com/item?id=25372336,
In that case, they could contact Google and in the end didn't need to pay.