I think you actually need to go through a third-party to really find out that information, and there does not seem to be a standard pricing. It sounds like support for the DRM requires the Reader Mobile SDK. From their site
The Adobe Reader Mobile SDK is available through a license agreement to OEMs, ISV integrators, and mobile applications developers on a case-by-case basis. For further information regarding licensing, please contact one of our reselling partners below:
At which point they list contact info for Datalogics, BSquare and Bluefire... Interesting that you negotiate the license from people producing software you would compete against, but I guess that would be the case even if you worked with Adobe directly.
Having said all that, I do want to reiterate that some/many of us won't really care about Adobe support. For myself, the few protected books I purchased last year were mostly on Comixology, and Adobe wouldn't really help there.