Flight to Siem Reap (Ho Chi Minh City)

Hi All,

I will be traveling in Vietnam from December 1, and I will have 12 days to go around. I'm thinking of going to Cambodia as well to see Phnom Penh and Siem Reap. The flights I see online are ridiculously expensive. Is there a cheaper alternative?

I read from some travel forums that it's cheaper buying flights to Cambodia, as well as domestic ones, when already in Vietnam. Is that true? I'm afraid I don't have much time to avail a cost-effective flight as I plan to go to Cambodia by December 5.

I'd appreciate all ideas and recommendations from all of you. TIA!

Angela