A cheaper & potentially slightly safer route to Siem Reap is to take the train from BKK to Aranya Prathet (48 bht, 6 hours), followed by a Tuk Tuk to the border (80 bht), then share a taxi with some other tourist to Siem Reap (say 50% of $40).

At the moment, the road is un-made most of the way, so this keeps speeds down, which must be a good thing.