O365, indeed, does not charge for outbound data. However, you are right that express route will charge outbound data unless you take an unlimited data plan. If you are using express route, then costs for IaaS / PaaS / O365 are handled in the same way.
As regards scalability, then we are in a beta stage right now and cannot provide exact numbers on what scalability and performance limits will be. Me personally, I don't use expressroute, so my outbound data costs are none, and the traffic today is acceptable, but I am not backing up a lot.
When we GA, we should be able to share some more data on the scalability limits.