We bought a bus ticket from Hoi An to Hue in our hotel (Vaia Hotel) and we were told that it wasn't a sleeping bus. Since we traveling in the morning (8 am) and the trip is not more than 4h we believed it. We paid $7 for it.
When we arrived, there was a sleeping bus waiting for us.
The others tourists had the same info (no sleeping bus). All of them paid different prices. A couple paid $4, a family from Malaysia paid $5 and other ladies paid more than $7.
A girl told us that during the night these buses are even worse because they put lots of mattresses on the floor, around the seats, to charge for more tickets. And there is no space or privacy.