Toby, RelayHealth does not integrate w/ AC (even though they reportedly attempted to contact AC because they would like to). I don't believe AC integrates with any portal at this time.
I really cant comment on the cost since we have a couple of extras since we were "first movers" in the area and have some services for free that others have to pay for. Like e-prescribing. But expect $250 - $300/year for basics.
So to eRx, you end up prescribing both in AC and in Relay. We've been experimenting with eRx in the new AC. It avoids the double entry, but Relay does better tracking/saving of what happened to your eRx, tells you that it was sent, and sends an automatic email to the patient that the Rx was sent to PHARMACY XYZ.
Oh, eRx is not one of the basics. I'm not sure at this time what would be included in the basic price since we've had it for 3 years or so now and it has changed.