It sounds like you're consuming a web API, so I think it doesn't matter if there's PHP, MySQL or whatever in the back. The data transferred via http at the end. As Javier mentioned the 'content-length' of the response header is probably the most important value [1].
Maybe it's a good idea to search for some AWS docs which describe how they calculate the costs. Olaf [1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Length -- Sent from: http://apache-flex-users.2333346.n4.nabble.com/
