when in doubt...
http://wikitravel.org/en/Hanoi
"There is a fixed price taxi stand right outside the airport exit, offering fares between US$10=16 into the city. It is slightly more expensive than the tout taxis, but you pre-pay for the trip, so no hassles about the fare. Other drivers may try to take you to a hotel of their choice to collect a commission, but if you are specific about your destination, they usually give in."
~Ray