Was in Sihanoukville last year during the off-season so prices may differ.
I got a simple large room, double bed for about $7 a night and motor-scooter rental for $6 a day. Petrol was so cheap as to not even worth mentioning the price. You can scooter around to all the beaches in the area. Just watch out for the police checkpoint/bribery post on the main roundabout. You don't have to go through it, easy enough to navigate around it by going down the side streets and entering the roundabout by another entrance.
I didn't personally find much of interest happening there during the off-season. I'm not into drugs or picking up dodgy hookers. I did have quite a pleasant time zipping around on the bike and getting stories from some of the expats in the bar area that have made their home there.
Ronald McDonald never sells to children. He informs and inspires through magic and fun.