It's probably true that you can't get a BKK to Siem Reap round trip airfare for less than $320 or so, because Bangkok Airways has a monopoly on the route.

But, if you are doing any OTHER side trips out of Bangkok in Thailand, you can save quite a bit by purchasing a Bangkok Airways discovery Airpass. The main caveat is that you need to buy the ticket while you are still within the U.S., and you need to purchase a minimum of 3 legs, up to 6. I personally used this to get a great deal on both BKK to Siem Reap, and BKK to Koh Samui routes, both of which are expensive, Bangkok Airways monopoly routes. By using the AirPass, I paid about the same for BOTH trips combined as either one would have cost me independently (about $350). Again, you can only buy this in the U.S. either from a U.S. Bangkok Airways ticket office, or a travel agent - not on line.

http://www.bangkokair.com/en/airpass.php#

FYI, my experience is that if you hire a guide and a driver, you can do the main Angkor temples (Angkor Wat, Angkor Tom, Ta Prohm, and some of the other nearby temples) that are close to Siem Reap in one long full sunrise to sunset day. Another day would be useful for seeing anything else in Siem Reap, such as the landmine museum and the floating village on Tonle Sap, or seeing some of the more far removed temple sites.