If you can pay up-front, you could get yourself a decent room for US$150 a month, or even less. See S Villa for some bargains (for example, a whole house for US$100 a month).
To find your way around town, familiarize yourself with the landmark and symbol of Sihanoukville: the Golden Lions. The Lions are a good place to meet girls!
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Thanks to social networks and the Internet, visitors to Sihanoukville have a lot more options these days than merely staying in a hotel. You could take part in a cultural exchange program, crash on somebody's couch, or share a room with a local. Couch Surfing (or staying at Utopia) is free; at Airbnb you have to pay. You will, however, end up with something a bit more substantial than a sofa/open air bench. For US$250 a month you could get your own house, complete with A/C and TV.
Cool Banana Bungalows: website here.
Says Auf Wiedersehen England on Get Jealous: "Opposite the Cool Banana Bungalows is the Utopia Guesthouse where we spend most afternoons,having a 0.50 USD beer at the pool, looking at the internet and playing cards."
Cool Banana apparently has its own mini-cinema playing Western movies.
Singles seem to cost US$8 a night, or for US$14 you can get four people under a roof for the night.
Monkey Republic: . Wesbite: website here.
Wooden strawroofed bungalows, set around a tropical garden, from US6-10 per night. The bungalows have a night security guard and are only 200m from Serendipity beach. On their Facebook page they are friends with Monkey Island on Koh Rong.
Utopia: . Phone: +855 (34) 934-319. Website: website here.
Free dorm beds are part of the attraction at this legendary Snooky establishment. If you insist on paying for your accommodation, a single room with fan and private bathroom costs US$5 a night. This place is famous for its nightlife, and the website promises three parties here a week. You can secure a glass of Anchor Beer for US$0.75 a glass, US$3 a for a jug, and US$7 a tower; there is a two-for-one deal for Happy Hour (9-10pm).