I think I read in past reviews that this museum is free. If that was the case, it is no longer free now. There is now a $5 fee per person to enter which, considering the fact that everything inside is just battlefield rubble, is way overpriced. If the cost was a dollar, maybe two, it would probably be appropriate...
We think you are confused with the Landmine Museum, here are the reasons why:
1. In the past the Landmine Museum had a free entrance. However, the War Museum Cambodia never had a free entrance.
2. The War Museum is just 5 km from Siem Reap, while the Landmine Museum is 25 km from the city.