Bangkok Airways have always been in the rip off business, just look at the fares to Samui. (Fly to Surat Thani and take the coach from there to the island) When Siem Reap opens up to budget airlines that will be a great improvement.

Meanwhile Lao Airlines flies daily from Vientiane to Siem Reap by way of Pakse. Still expensive at $130, and $226 if you add Chiang Mai to Vientiane; you also need an overnight in Vientiane.