I'd guess around $800 for the 3 year visa (or around $550 if you can get the mythical "official price")
http://www.khmertimeskh.com/news/28689/ ... ext-month/The three-year multiple-entry visa, a new government strategy to encourage foreigners to come to the Kingdom, will allow foreigners to stay in the country for up to three years, Minister of Tourism Thong Khon said on Wednesday.
The multiple-entry visa application will be available from September 1 to foreigners from all countries, Mr. Khon said, with different prices according to the length of stay although pricing details of the new visas were unavailable.
The new entry visa will allow tourists and business people to enter and exit Cambodia many times during the visa’s validity period, Mr. Khon said.
“They can apply for a visa in Cambodia and can travel in and out many times according to the visa they apply for. Tourists and investors can apply for a two-year stay or three-year stay with multiple entries.”