You guys should really retest with it off. Might save a bunch of money. :)
No CDN that I have used (and we've used Panther Networks, CDNetworks, Cotendo, EdgeCast, Akamai and Fastly) has ever charged us for ingress.
Usually you pay for egress from edges and whatever is going between the edge and origin.
So it would be good to get that clarified so we know.