It IS quite a drive from town, in the Angkor Archaeological Park towards the Banteay Srei (one of the temples that's further away from the main ones) but not THAT far. Seriously.
I arranged transportation with the hotel for US$23. You should be able to negotiate a cheaper rate if you take a tuk tuk from town.
Entry fee - US$5 per adult
Note: This place is different than the war museum in town. Some locals might suggest that you to go to the war museum instead, as it is nearer to town.
I LOVE this place.
I learned a lot about the war in Cambodia and the general horror of landmines all over the world.