Another option of saving money is to do the trip via Kuala Lumpur. Infact Air Asia has a flight from KL to Siem Reap.
I checked because I want to go next year, and for two persons, Phuket-KL - Siem Reap is 12,000 bath for two persons.

However you must stay one night in KL both when going and when coming back. I don't know if from Bangkok is the same. Well, going to SR from BKK via KL sounds crazy lol....

Ahhh I'm really kiniaw