From Hong Kong I think it is expensive - might be cheaper to book a package including hotel. Or, getting an Bangkok Airlines airpass might be the way.
I went to Cambodia with a tour company in May & it cost around $6500, but my friend went later with just a group of friends and they were fine just walking around by themselves.
Well, Siem Riep is very popular with tourists and there's LOTS of hotels near the area...Phnom Penh on the other hand is not so safe..
That is unfortunate, a friend and I flew Singapore Airlines (HKG-SIN-HKG) and Silkair (SIN-REP, PNH-SIN) for HKD 1,700 in total in February soon after the CNY holidays.
Zuji gave us a guide as to the cheapest way to get there (advertised SQ as the lowest fare) and from there we checked on the SQ website and that's where we purchased our ticket.
Perhaps we were just lucky.