There is a lot of countries, and Cambodia is not an exception, where it is possible to negotiate with the girl and try to go with her for less money, but it is considered as a mauvais ton, and to get paid less than twenty mean now to lose the face and respect.
Usually, twenty dollars price is only for Khmer people and the foreigners, long time living in Cambodia.
People looking for the same may approach those working girls for having sex or in order to avail any other sexual services that might be available.
The Mototaxis and Tuc Tuc drivers want ALWAYS commission from the bar/shop owners AND from customer! Angry blood with driver, customer and bar/massage owner is sure!
If you are in Sihanoukville, is much better and more easy for you, if you rent for your holiday a scooter for 6 to 7 dollars per day.
Meth is also known as "ice" or "Jabba" and it's a quite cheap drug what locals like to smoke or sometimes sniff.
The drug is highly, highly addictive, but allows taxi girls to stay up working throughout the night and the morning.