It definitely varies wildly. Some methods of decaffeination might leave as much as one-third of the coffee still in the beans.
I'm a regular coffee drinker (and not caffeine sensitive), and I do get a placebo boost from decaf, but without any noticeable effects from actual caffeine.
I’ve heard that sometimes ordering decaf will get you a caffeinated drink and that a lot of places disregard or don’t pay enough attention to that particular request. I have no source or any numbers though.
That's why I always order instant decaf so I can look at the packet before dissolving it.
That's a smart thing to do - just ask for hot water at starbucks (if you're out of the house of course) and slip in a decaf tube. It would probably be free.