If your overstay is less than 30 days it's $5 else it's $6. (just because the maximum extension for a Cambodian visa is only 1 month), if you're gonna stay more then 8 days longer, I'd suggest you do a quick visa run.
We had a ten day overstay in Cambodia and it was 5$ a day - about the same price as an extension and less hassle than a visa run. Actually worked out well at the border as when we approach an offical (rather than joining the immigration queue) and he realised he was about to get 100$ from us he bought up chairs for us to sit on while he went away and returned with our passport & exit stamps - didn't have to queue at all and just walked straight through!!!