It's been hard to measure since turning on Argo coincided with a larger rewrite of the backend. Overall performance is up but the real culprit might have been more of the optimizations on the origin.
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.