Khao Na Phet is a famous duck dish in Thailand. It’s actually roasted duck sliced on top of rice. It’s normally served with a soup (clear broth) some green vegetables and marinated ginger. A black sauce is also a must on your duck.
Price may very but duck is more expensive than chicken in Thailand. That dish is normally 40 to 60 baht (Inflation is so high that I am not sure right now). You will recognize these places with roasted ducks hanging in front of their stand or restaurants. I like that because it reminds me Chinese food (actually, Peking Duck is much better!) and it is not spicy. Perfect to change from Thai food once in a while at cheap price.
My favorite place is beside Monkey Bar on Suranaree street, on the sidewalk. You might prefer another one, but this one is just convenient. In the center of the city, opened at night, etc. I do not have a picture of the place but you can’t miss it if you know Monkey Bar. Same side of the road, maybe 200 meters, toward the Memorial hospital.
I also really like the one just on the corner of Wat Nong Cha Bok, on Mukkamontree road. That place has more dishes and it’s quite popular for lunches. I do not think it is opened at night. It is considered as a Chinese restaurant by Google. Location : https://goo.gl/maps/uCPR5mzXNM8qY1Es6
Finally, a place less known is near Wat Pha Salawan. But be careful, it is so popular that if you go after 2pm, it might already all be gone! They also do the red pork (called Moo Deang) and others.
You can also find this dish in food court, like Terminal 21, the Mall or Saveone market. But these 3 places are quite famous for their duck.