I'm setting up Amazon Cloudfront in front of (maybe in place of) varnish and I want to estimate the cost.
I can consider the sum of the fields s_bodybytes and s_hdrbytes of varnistat as monthly traffic to be indicated in the "aws simple monthly calculator" (http://calculator.s3.amazonaws.com/index.html) for CloudFront ? (obviously calculated in a correct time period)
Yes, those appear to collectively represent total bytes sent to clients, so that should be a reasonable approximation of the amount of "out to the Internet" traffic for CloudFront usage charge estimation.
There are other charges, a per-request charge and a charge for upstream bandwidth between CloudFront and the origin server, but those should be negligible in comparison, for most applications.