Angkor Air is cheap BKK-REP (Siem Reap) $60 (ow) -
http://www.cambodiaangkorair.com/en/home.aspx. Getting to Koh Samui isn't. Maybe look into a Bangkok Air 'Discovery Airpass' -
http://www.bangkokair.com/pages/disc...covery-airpass. Taking a cab or minivan to Koh Chang from BKK (4 or 5 hrs) is also a good option. There are flights to Sihanoukville from REP a couple days a week too.