BangkokAir has a Discovery Pass which costs, BKK to Siem Reap $90,

Siem Reap to Phnom Penh $55 and PP to BKK $90 Total $235.

The seat class is lower than economy which means you must wait behind everyone else if you change a booking and want to go standby if the plane is full.

But the BangkokAir Club facilities are so nice.....