is Bangkok Airways/Siem Reap Air the only airline to fly BKK-SR?

Bangkok airways is the only airline that does this route.
Another option is to fly with airasia to Phnom Penh then take the 10.30am Capitol bus to Siem Reap as airasia flights are only once daily and it arrives in Phnom Penh around 8am. You will arrive in Siem Reap around 3.30pm this way.
The other option is overland and you will find up to date information on how to do this here

Yes, they have a cozy monopoly agreement with the Cambodian Government for the next few years...

Hi,
When I do a search on this on Bangkok Airways the rate that comes up is 11,235 baht, or roughly $320. That seems like a ton! Is that correct? Or is there a cheaper alternative?
Thanks!
