Of our two upcoming July cruises - the Sensation 4 day (over July 4th weekend) and the Glory (7 day July 19th) - the rates on the Sensation dropped $200 pp for a 6a (from $573 to $373) when the military discount was added. I seem to recall on the Glory we received about $50 off per person (Cat 8a)...
Carnival told me it really depends on which cruise, when, and how booked it already is -that determines the true value of the discount - so it can change daily... just like their standard fares.
... and by the way, is it only me, or do others see no point in paying more for a "higher cat" - from 8a to 8d for example.... it is the exact same room, simlpy a different deck...!
Tom and Cheryl