I just been there recently. Maybe you would like to read what I type here:
From Khao San Road, this is the way they manage it.
It is quite cheap to travel from Bangkok to Siam Reap, Cambodia. They have quite a good road system in Thailand but not after the border between Thailand and Cambodia. They will use van/ mini bus to send you from Khao San, Bangkok to the border. After that, (be careful when you cross the border, lot of pickpockets and beggars) you need to walk quite far to a nearer shop to wait for the transport from the border to Siam Reap (they separate the transport into 2: Thailand and Cambodia). In Cambodia, at most they will use minibus to send you all to Siam Reap.
There is a catch:
Normally, they will cooperate with each other that they send you all to their own guesthouse in Siam Reap. (from what I have found, one or two dollars more expensive per day, and for motor ride, around few dollars more expensive per day). So, after a whole day (7 to 8 hours) bumpy road trip, NEVER yield to them. Don't worry, cause Siam Reap town is quite small, not so big. Most of the guesthouses are within short walking distance. I have checked already, the one I stayed, ripped us off a bit (S***!). and a bit farther from the town and that one is not far from another guesthouse.
One thing you can do is just stay one night in any guesthouse you can find at the first night.
The second day, try to wake up earlier and bargain with other guesthouses and get the price you want. Normally, since they want businesses (it is a lot of $$$, man, cause mostly you will stay there, hire tourguides there and so forth) therefore they will give you some good prices!
Hope you can have a nice trip.