Nationals of European Union Countries, USA, Russia, China and Asean Countries usually receive a tourist visa on arrival, and this is valid for one month. It costs 20  US Dollars and it can be extended for a further month for the cost of around 30-40 Dollars ( depending by where you apply for that).
A business visa goes for 25 Dollars, it is valid for one month and can be extended indefinitely, you got the money, they got the stamp. Extensions are 40 bucks for one month, 80 Dollars for 3 months, six months go for 155 Dollars while a one-year extension is now at 300 Dollars.
Bring a couple of passport-size photos with you when you arrive.
Seems there is no limitation on the amount of cash you can bring into the Kingdom, as in a developing Country money is always the most welcome. Don't waste your money buying booze and cigs at various duty-free shops , as you can find such items cheaper here than elsewhere. A Marlboro carton goes for 9-10 bucks in a normal Cambodian marketplace.