If you have the time, you can do BKK-PNH (can be very inexpensive on AirAsia) and then hire a driver to go from PNH to Siem Riep (~4-5 hrs, ~$75 for a trip last Xmas). I would only suggest doing this if you are staying 1-2 days in Phnom Penh (which I really enjoyed and am glad that I didn't skip).