The US Embassy Phnom Penh website said the same thing, with a $5 per day fine for 30 days or less and $6 per day for over 30 days. As far as I can tell, the information on the website is current.
Where are you now alt? It is fairly easy to extend a visa in Thailand, so I wonder if it is just as easy in Cambodia. Either way, will you let us know how it works out - extension process or fine - so we've got recent info? Thanks.