They quote the price for the shuttle USD, but they make foreigners pay the same amount whether they pay in dong or USD. So $2 is about 38,000 dong. They charge foreigners paying in dong 38,000, but they charge locals 30,000. I pushed back both times and it made no difference once, and the other time they gave me change so I only paid 35,000.

That said, most of the other foreigners on my shuttle did not realize there were different prices for locals, as they do not make it obvious. However the local/foreigner price difference is posted on most sites:
http://www.hanoiairportonline.com/hanoi-airport-minibus
http://hanoiairporttransfer.com/en/T...portation.aspx

Anyway, if you're cheap it's a good option. If you're not, take a taxi.