These balloons are so common now in Hanoi. Kids love them because there are heaps of different colours and shapes, they're made from plastic and they can be kept for a long time. Lots of people exchange balloons as simple gifts instead of other stuff and they also use them to decorate for christmas, weddings and birthday parties.
I asked the seller and she offered me one for 20,000VND but the couple before me paid 10,000. As I mentioned in some of my other posts, bargaining is just a game that should not be too serious.