Considering a single OEM horn is $97-$110 retail at a Nissan dealer, depending on the horn, although likely unavailable, I don't see how $15 is a lot of money? Unless you left out a zero on the end.
Then again, the stock horns sound pathetic and I can't imagine why, other than originality, you'd want new OEM ones. I had planned on going Hella, they sound pretty decent.