I'm extrapolating up to 20th, since my highest level guys are 16th and 17th, but... my cleric is currently about AC 65 and I don't see that really changing at 20th unless new material is released. My dervish's AC would go up by a few points due to the Dervish AC bonus and Improved Combat Expertise. So if I want to totally sacrifice offense with him he could get up to 76, plus Dodge and Mobility.
Getting a high AC at 20th level isn't really all that hard if you want to put a lot of effort into it. I'd say that while they can get their ACs up that high, they are usually running an AC of about 40-55 for most fights.