There is no bus going directly from Siem Reap to Preah Vihear.
You can get a private taxi to do the trip in a day for USD 80 to 120 to & back for a 4-door passenger car taxi depending on your negotiation skill, or you can pay USD 6 to 8 a person each way for a shared car or van.
However, these shared taxis usually pack up to 7 passengers in a 4-door passenger car or up to 20 passengers in a 10-seat van, excluding the driver! Moreover, the driver will often drive around to look for more passengers before heading off to Preah Vihear.
G.S.T. Bus operates 2 buses daily from Siem Reap to Anlong Veng at USD 5 a person each way for the 2-hour journey. From Anlong Veng, it’s about 100km to the ticketing office of Preah Vihear.
A private 4-door passenger car taxi between Anlong Veng and Preah Vihear ticketing office is USD 35 to 40 for the 2-way journey, including having the driver wait 3 hours during your visit. The cost of a motorcycle taxi for the same journey is USD 10 to 15.
Ritho Bus operates between Anlong Veng and Sra Em which is near Preah Vihear. Ritho Bus is directly across G.S.T. Bus in Anlong Veng.