Originally Posted by CrazyInteg

It's more than 10 minutes, unless Tran Andretti is driving.

(You're wanting a bus to Hai Phong city) It's likely cheaper to book the whole day trip through a company that does these tours. They usually include all transportation. Keep in mind that 99.9% of the people going there are tourists (a.k.a. not local Hai Phongians). The tour will pick you up in Hanoi, take you around all day, and bring you back to Hanoi in the evening.

I understand what you're after. Absolute cheapest way will not be the fastest way and be very inconvenient. On top of that there is a language barrier.
You haven't even started asking us, "Which Halong Bay cruise should I take?"

My advice to you is to take a taxi into the old town after your flight arrives. The only other option is to get a shared bus from the 1st or 2nd curb off the main walkway. You will need to know the price beforehand and negotiate to get that price. It might take walking away 2 times before they agree to the correct price. (Don't pay over 100k Dong per person).
You will spend hours researching the dozens of Halong Bay cruise ships. You will get frustrated. Then you will hear about Lan Ha and get even more frustrated.

Spending an hour of research to find a cheap ride to town or cheaper airport hotel to save $10, can easily be lost in a Halong Bay cruise that costs $60 per person.