Where in the description does it say the +3 versions last for minutes? I just looked it up, and the only duration listed is that each dagger conjured lasts only 3 rounds.
Also, with it being a swift action to make the dagger appear, you cannot make a full attack action by throwing a bunch of daggers from the thing-- only 1 per round-- so I don't think the item is overpriced.
Essentially, for 9 rounds a day, you get a +3 weapon for little less than the cost of a +2 weapon, and a free dagger to throw each round.
However, where this could get broken is with the Flick of the Wrist feat.