I seem to remember a friend coming off a cruise with 30+ bottles of booze (not wine) and the duty was charged by only a few $ each bottle, nothing too large.

Then other friends told some sort of story of arriving at IAD (Washington Dulles) and their excess wine being held and released to a distributor or something – so that was a hassle – but likely that related to a Virginia law. Here is a good discussion that is Dulles related: http://chowhound.chow.com/topics/261455 That link states the duty is 10%, who knows how the establish the value.