$60-70 is the going rate for a day trip (round trip) from Danang to Hue, including waiting time.
As the airport is in Danang, you should stay there. See the Marble Mountains, Lady Buddha statue and Non Nuoc Beach, and then get a car and driver to Hue. 2hr drive. $70 max. Come back the same day, as only 3 tombs need to be looked at, not all of them.
Next day, Danang to Hoi An, roughly $35 round trip in a private car, maybe more if you include My Son on the way back.
Final night in Danang before flying onwards (for me I flew to Siem Reap on Silk Air).