Bus ride

I 'd back from Hoi An and my case may have st good for you.  There only bus from airport to the center of DN city and then you can tell the bus driver to drop you at Lê Duẩn street ( ofcourse at the bus station) and then you catch another bus to Hoi An - the number 1 about 25000vnd and from airport may be the same price. You may have to walk to find bus station at airport but can ask the airport staff for help,  don't ask taxi driver.
Taxi from airport to center about 100. 000vnd.
If you don't like go by simple bus to Hoi An when you in center you can go by high qualitive bus as Phương Trang,  the Sing tour or Hanh cafe,  they ' re easy to find on the website,  price about 80-100. 000 vnd.  These buses more comfotable.
Normal taxi from DN airport to HA cost 300. 000vnd,  but I booked a taxi of website taxiairport. vn it was 200 . 000 vnd and go directly to HA ( 1 hour).  You have to book taxi a day before and if you go alone you can order (on website) to share with someone go the same way so the price can down to 100. 000vnd. 
I'm not sure they service the way Hoi An - DN or not but you can go by high qual bus I said above or just continue to Hue city by bus .
Oh I was cheated by mailinh taxi and vinasun taxi,  may be just of 2 drivers but I had bad impression and advice that you should go by Tiên Sa taxi,  airport taxi ok too. 
Good luck.

Thanks a lot Bngoc. Very helpful info.