Airfare BKK-Siem Riep-BKK on Bangkok Airways is listed at about US$300. See
http://www.cambodia-travel.com/cambodia-airfare.htm
I've used Royal Exclusive Travel in Bangkok
http://www.royalexclusive.com/ but there are a lot of alternatives in the Khao San Road area, one of whom, Olavi Tours, lists a return fare as THB8800, or about USD$200.
http://www.olavi.com/Asia.htm I don't have experience with this firm.
Take a look at
http://www.khao-san-road.com/travel.html
Rich