AppEngine features a member in its logs called cpm_usd. As far as I understand this is the approximate cost of 1000 of such requests in US Dollars.
Since 08/16/2012 these numbers are SIGNIFICANTLY smaller (factor 500) for my app (I did not change my app). I was wondering what this is about?
Did Google change the way they calculate those costs?
Are frontend hours included or does this only include calls to services like the datastore?
The only answer I have is that they stopped including the frontend hours in the calculation (I am currently still in dev mode and am thus accumulating a lot of idle time that could have distorted the original / old result).
I am not sure why your numbers have changed but my understanding is that, as of the pricing changes last year, this number is not relevant anymore.
Around the time your numbers changed Google was adding cost tracking functionality to the AppStats tool. What you can do now is turn on pricing metrics in AppStats and get an accurate picture of the RPC costs of your request (which covers pretty much all costs except instance hours).
A quick test of a few requests on one of my apps shows that the cpm_usd and the cost reported by AppStats are not in line at all. Based on the number reported by cpm_usd for the requests I was just testing there is no way that number could contain datastore costs which means it is basically useless to me.
Check out the cost tracking that AppStats can provide and see how your own numbers line up:
Update Sept 5, 2012:
I asked about the current relevance of cpm_usd in a recent App Engine office hours hangout and while they could not give an exact answer they indicated they think it is still a relevant number. It would be nice to have more insight into what cpm_usd currently represents. Here is a recording of Amy answering the question: