We booked a tuk tuk through our hostel for $15 for three people. He took us to the museum first where we spent three hours. We paid $8 each including the audio.
The museum was laid out really well and the audio was really well done and informative. Without the audio you are just looking at a building and some pictures. It's really worth paying the extra for the audio even though it is a lot.
Second stop was the killing fields where we paid $6 each and audio was included; this took us a total of two hours. The audio was really informative and gave me an eerie feeling walking in the footsteps of both Khmer Rouge and their victims. There are still fragments of bone laying on the ground which come up in rainy season and only get collected every few weeks.
It's a really heavy and tiring day so bring water with you. I had to have an afternoon nap when I got back to the hostel. However, the way they have laid out the museum and the audio were both incredible. I'm really impressed with how well they are telling this story.